Merge branch 'fr/vimdiff-layout'

Reimplement "vimdiff[123]" mergetool drivers with a more generic
layout mechanism.

* fr/vimdiff-layout:
  mergetools: add description to all diff/merge tools
  vimdiff: add tool documentation
  vimdiff: integrate layout tests in the unit tests framework ('t' folder)
  vimdiff: new implementation with layout support

27 files changed, 978 insertions, 32 deletions

diff --git a/Documentation/Makefile b/Documentation/Makefile
index 44c080e3e5..adb2f1b50a 100644
--- a/Documentation/Makefile
+++ b/Documentation/Makefile
@@ -302,12 +302,12 @@ $(mergetools_txt): mergetools-list.made
 
 mergetools-list.made: ../git-mergetool--lib.sh $(wildcard ../mergetools/*)
 	$(QUIET_GEN) \
-	$(SHELL_PATH) -c 'MERGE_TOOLS_DIR=../mergetools && \
+	$(SHELL_PATH) -c 'MERGE_TOOLS_DIR=../mergetools && TOOL_MODE=diff && \
 		. ../git-mergetool--lib.sh && \
-		show_tool_names can_diff "* " || :' >mergetools-diff.txt && \
-	$(SHELL_PATH) -c 'MERGE_TOOLS_DIR=../mergetools && \
+		show_tool_names can_diff' | sed -e "s/\([a-z0-9]*\)/\`\1\`;;/" >mergetools-diff.txt && \
+	$(SHELL_PATH) -c 'MERGE_TOOLS_DIR=../mergetools && TOOL_MODE=merge && \
 		. ../git-mergetool--lib.sh && \
-		show_tool_names can_merge "* " || :' >mergetools-merge.txt && \
+		show_tool_names can_merge' | sed -e "s/\([a-z0-9]*\)/\`\1\`;;/" >mergetools-merge.txt && \
 	date >$@
 
 TRACK_ASCIIDOCFLAGS = $(subst ','\'',$(ASCIIDOC_COMMON):$(ASCIIDOC_HTML):$(ASCIIDOC_DOCBOOK))
diff --git a/Documentation/config/mergetool.txt b/Documentation/config/mergetool.txt
index cafbbef46a..90b3809700 100644
--- a/Documentation/config/mergetool.txt
+++ b/Documentation/config/mergetool.txt
@@ -45,6 +45,15 @@ mergetool.meld.useAutoMerge::
 	value of `false` avoids using `--auto-merge` altogether, and is the
 	default value.
 
+mergetool.vimdiff.layout::
+	The vimdiff backend uses this variable to control how its split
+	windows look like. Applies even if you are using Neovim (`nvim`) or
+	gVim (`gvim`) as the merge tool. See BACKEND SPECIFIC HINTS section
+ifndef::git-mergetool[]
+	in linkgit:git-mergetool[1].
+endif::[]
+	for details.
+
 mergetool.hideResolved::
 	During a merge Git will automatically resolve as many conflicts as
 	possible and write the 'MERGED' file containing conflict markers around
diff --git a/Documentation/git-mergetool.txt b/Documentation/git-mergetool.txt
index e587c7763a..f784027bc1 100644
--- a/Documentation/git-mergetool.txt
+++ b/Documentation/git-mergetool.txt
@@ -101,6 +101,7 @@ success of the resolution after the custom tool has exited.
 
 CONFIGURATION
 -------------
+:git-mergetool: 1
 include::config/mergetool.txt[]
 
 TEMPORARY FILES
@@ -113,6 +114,13 @@ Setting the `mergetool.keepBackup` configuration variable to `false`
 causes `git mergetool` to automatically remove the backup as files
 are successfully merged.
 
+BACKEND SPECIFIC HINTS
+----------------------
+
+vimdiff
+~~~~~~~
+include::mergetools/vimdiff.txt[]
+
 GIT
 ---
 Part of the linkgit:git[1] suite
diff --git a/Documentation/mergetools/vimdiff.txt b/Documentation/mergetools/vimdiff.txt
new file mode 100644
index 0000000000..2d631e9b1f
--- /dev/null
+++ b/Documentation/mergetools/vimdiff.txt
@@ -0,0 +1,194 @@
+Description
+^^^^^^^^^^^
+
+When specifying `--tool=vimdiff` in `git mergetool` Git will open Vim with a 4
+windows layout distributed in the following way:
+....
+------------------------------------------
+|             |           |              |
+|   LOCAL     |   BASE    |   REMOTE     |
+|             |           |              |
+------------------------------------------
+|                                        |
+|                MERGED                  |
+|                                        |
+------------------------------------------
+....
+`LOCAL`, `BASE` and `REMOTE` are read-only buffers showing the contents of the
+conflicting file in specific commits ("commit you are merging into", "common
+ancestor commit" and "commit you are merging from" respectively)
+
+`MERGED` is a writable buffer where you have to resolve the conflicts (using the
+other read-only buffers as a reference). Once you are done, save and exit Vim as
+usual (`:wq`) or, if you want to abort, exit using `:cq`.
+
+Layout configuration
+^^^^^^^^^^^^^^^^^^^^
+
+You can change the windows layout used by Vim by setting configuration variable
+`mergetool.vimdiff.layout` which accepts a string where the following separators
+have special meaning:
+
+  - `+` is used to "open a new tab"
+  - `,` is used to "open a new vertical split"
+  - `/` is used to "open a new horizontal split"
+  - `@` is used to indicate which is the file containing the final version after
+    solving the conflicts. If not present, `MERGED` will be used by default.
+
+The precedence of the operators is this one (you can use parentheses to change
+it):
+
+    `@` > `+` > `/` > `,`
+
+Let's see some examples to understand how it works:
+
+* `layout = "(LOCAL,BASE,REMOTE)/MERGED"`
++
+--
+This is exactly the same as the default layout we have already seen.
+
+Note that `/` has precedence over `,` and thus the parenthesis are not
+needed in this case. The next layout definition is equivalent:
+
+    layout = "LOCAL,BASE,REMOTE / MERGED"
+--
+* `layout = "LOCAL,MERGED,REMOTE"`
++
+--
+If, for some reason, we are not interested in the `BASE` buffer.
+....
+------------------------------------------
+|             |           |              |
+|             |           |              |
+|   LOCAL     |   MERGED  |   REMOTE     |
+|             |           |              |
+|             |           |              |
+------------------------------------------
+....
+--
+* `layout = "MERGED"`
++
+--
+Only the `MERGED` buffer will be shown. Note, however, that all the other
+ones are still loaded in vim, and you can access them with the "buffers"
+command.
+....
+------------------------------------------
+|                                        |
+|                                        |
+|                 MERGED                 |
+|                                        |
+|                                        |
+------------------------------------------
+....
+--
+* `layout = "@LOCAL,REMOTE"`
++
+--
+When `MERGED` is not present in the layout, you must "mark" one of the
+buffers with an asterisk. That will become the buffer you need to edit and
+save after resolving the conflicts.
+....
+------------------------------------------
+|                   |                    |
+|                   |                    |
+|                   |                    |
+|     LOCAL         |    REMOTE          |
+|                   |                    |
+|                   |                    |
+|                   |                    |
+------------------------------------------
+....
+--
+* `layout = "LOCAL,BASE,REMOTE / MERGED + BASE,LOCAL + BASE,REMOTE"`
++
+--
+Three tabs will open: the first one is a copy of the default layout, while
+the other two only show the differences between (`BASE` and `LOCAL`) and
+(`BASE` and `REMOTE`) respectively.
+....
+------------------------------------------
+| <TAB #1> |  TAB #2  |  TAB #3  |       |
+------------------------------------------
+|             |           |              |
+|   LOCAL     |   BASE    |   REMOTE     |
+|             |           |              |
+------------------------------------------
+|                                        |
+|                MERGED                  |
+|                                        |
+------------------------------------------
+....
+....
+------------------------------------------
+|  TAB #1  | <TAB #2> |  TAB #3  |       |
+------------------------------------------
+|                   |                    |
+|                   |                    |
+|                   |                    |
+|     BASE          |    LOCAL           |
+|                   |                    |
+|                   |                    |
+|                   |                    |
+------------------------------------------
+....
+....
+------------------------------------------
+|  TAB #1  |  TAB #2  | <TAB #3> |       |
+------------------------------------------
+|                   |                    |
+|                   |                    |
+|                   |                    |
+|     BASE          |    REMOTE          |
+|                   |                    |
+|                   |                    |
+|                   |                    |
+------------------------------------------
+....
+--
+* `layout = "LOCAL,BASE,REMOTE / MERGED + BASE,LOCAL + BASE,REMOTE + (LOCAL/BASE/REMOTE),MERGED"`
++
+--
+Same as the previous example, but adds a fourth tab with the same
+information as the first tab, with a different layout.
+....
+---------------------------------------------
+|  TAB #1  |  TAB #2  |  TAB #3  | <TAB #4> |
+---------------------------------------------
+|       LOCAL         |                     |
+|---------------------|                     |
+|       BASE          |        MERGED       |
+|---------------------|                     |
+|       REMOTE        |                     |
+---------------------------------------------
+....
+Note how in the third tab definition we need to use parenthesis to make `,`
+have precedence over `/`.
+--
+
+Variants
+^^^^^^^^
+
+Instead of `--tool=vimdiff`, you can also use one of these other variants:
+
+  * `--tool=gvimdiff`, to open gVim instead of Vim.
+
+  * `--tool=nvimdiff`, to open Neovim instead of Vim.
+
+When using these variants, in order to specify a custom layout you will have to
+set configuration variables `mergetool.gvimdiff.layout` and
+`mergetool.nvimdiff.layout` instead of `mergetool.vimdiff.layout`
+
+In addition, for backwards compatibility with previous Git versions, you can
+also append `1`, `2` or `3` to either `vimdiff` or any of the variants (ex:
+`vimdiff3`, `nvimdiff1`, etc...) to use a predefined layout.
+In other words, using `--tool=[g,n,]vimdiffx` is the same as using
+`--tool=[g,n,]vimdiff` and setting configuration variable
+`mergetool.[g,n,]vimdiff.layout` to...
+
+  * `x=1`: `"@LOCAL, REMOTE"`
+  * `x=2`: `"LOCAL, MERGED, REMOTE"`
+  * `x=3`: `"MERGED"`
+
+Example: using `--tool=gvimdiff2` will open `gvim` with three columns (LOCAL,
+MERGED and REMOTE).
diff --git a/git-mergetool--lib.sh b/git-mergetool--lib.sh
index 542a6a75eb..9f99201bcc 100644
--- a/git-mergetool--lib.sh
+++ b/git-mergetool--lib.sh
@@ -63,7 +63,7 @@ $(list_tool_variants)"
 					preamble=
 				fi
 				shown_any=yes
-				printf "%s%s\n" "$per_line_prefix" "$toolname"
+				printf "%s%-15s  %s\n" "$per_line_prefix" "$toolname" $(diff_mode && diff_cmd_help "$toolname" || merge_cmd_help "$toolname")
 			fi
 		done
 
@@ -162,10 +162,18 @@ setup_tool () {
 		return 1
 	}
 
+	diff_cmd_help () {
+		return 0
+	}
+
 	merge_cmd () {
 		return 1
 	}
 
+	merge_cmd_help () {
+		return 0
+	}
+
 	hide_resolved_enabled () {
 		return 0
 	}
diff --git a/mergetools/araxis b/mergetools/araxis
index e2407b65b7..eb32a7da95 100644
--- a/mergetools/araxis
+++ b/mergetools/araxis
@@ -2,6 +2,10 @@ diff_cmd () {
 	"$merge_tool_path" -wait -2 "$LOCAL" "$REMOTE" >/dev/null 2>&1
 }
 
+diff_cmd_help () {
+	echo "Use Araxis Merge (requires a graphical session)"
+}
+
 merge_cmd () {
 	if $base_present
 	then
@@ -13,6 +17,10 @@ merge_cmd () {
 	fi
 }
 
+merge_cmd_help () {
+	echo "Use Araxis Merge (requires a graphical session)"
+}
+
 translate_merge_tool_path() {
 	echo compare
 }
diff --git a/mergetools/bc b/mergetools/bc
index 26c19d46a5..2922667ddd 100644
--- a/mergetools/bc
+++ b/mergetools/bc
@@ -2,6 +2,10 @@ diff_cmd () {
 	"$merge_tool_path" "$LOCAL" "$REMOTE"
 }
 
+diff_cmd_help () {
+	echo "Use Beyond Compare (requires a graphical session)"
+}
+
 merge_cmd () {
 	if $base_present
 	then
@@ -13,6 +17,10 @@ merge_cmd () {
 	fi
 }
 
+merge_cmd_help () {
+	echo "Use Beyond Compare (requires a graphical session)"
+}
+
 translate_merge_tool_path() {
 	if type bcomp >/dev/null 2>/dev/null
 	then
diff --git a/mergetools/codecompare b/mergetools/codecompare
index 9f60e8da65..610963d377 100644
--- a/mergetools/codecompare
+++ b/mergetools/codecompare
@@ -2,6 +2,10 @@ diff_cmd () {
 	"$merge_tool_path" "$LOCAL" "$REMOTE"
 }
 
+diff_cmd_help () {
+	echo "Use Code Compare (requires a graphical session)"
+}
+
 merge_cmd () {
 	if $base_present
 	then
@@ -13,6 +17,10 @@ merge_cmd () {
 	fi
 }
 
+merge_cmd_help () {
+	echo "Use Code Compare (requires a graphical session)"
+}
+
 translate_merge_tool_path() {
 	if merge_mode
 	then
diff --git a/mergetools/deltawalker b/mergetools/deltawalker
index ee6f374bce..efae4c285c 100644
--- a/mergetools/deltawalker
+++ b/mergetools/deltawalker
@@ -2,6 +2,10 @@ diff_cmd () {
 	"$merge_tool_path" "$LOCAL" "$REMOTE" >/dev/null 2>&1
 }
 
+diff_cmd_help () {
+	echo "Use DeltaWalker (requires a graphical session)"
+}
+
 merge_cmd () {
 	# Adding $(pwd)/ in front of $MERGED should not be necessary.
 	# However without it, DeltaWalker (at least v1.9.8 on Windows)
@@ -16,6 +20,10 @@ merge_cmd () {
 	fi >/dev/null 2>&1
 }
 
+merge_cmd_help () {
+	echo "Use DeltaWalker (requires a graphical session)"
+}
+
 translate_merge_tool_path () {
 	echo DeltaWalker
 }
diff --git a/mergetools/diffmerge b/mergetools/diffmerge
index 9b6355b98a..9b5b62d1ca 100644
--- a/mergetools/diffmerge
+++ b/mergetools/diffmerge
@@ -2,6 +2,10 @@ diff_cmd () {
 	"$merge_tool_path" "$LOCAL" "$REMOTE" >/dev/null 2>&1
 }
 
+diff_cmd_help () {
+	echo "Use DiffMerge (requires a graphical session)"
+}
+
 merge_cmd () {
 	if $base_present
 	then
@@ -13,6 +17,10 @@ merge_cmd () {
 	fi
 }
 
+merge_cmd_help () {
+	echo "Use DiffMerge (requires a graphical session)"
+}
+
 exit_code_trustable () {
 	true
 }
diff --git a/mergetools/diffuse b/mergetools/diffuse
index 5a3ae8b569..ebfaba5172 100644
--- a/mergetools/diffuse
+++ b/mergetools/diffuse
@@ -2,6 +2,10 @@ diff_cmd () {
 	"$merge_tool_path" "$LOCAL" "$REMOTE" | cat
 }
 
+diff_cmd_help () {
+	echo "Use Diffuse (requires a graphical session)"
+}
+
 merge_cmd () {
 	if $base_present
 	then
@@ -13,3 +17,7 @@ merge_cmd () {
 			"$LOCAL" "$MERGED" "$REMOTE" | cat
 	fi
 }
+
+merge_cmd_help () {
+	echo "Use Diffuse (requires a graphical session)"
+}
diff --git a/mergetools/ecmerge b/mergetools/ecmerge
index 6c5101c4f7..0d4d609874 100644
--- a/mergetools/ecmerge
+++ b/mergetools/ecmerge
@@ -2,6 +2,10 @@ diff_cmd () {
 	"$merge_tool_path" --default --mode=diff2 "$LOCAL" "$REMOTE"
 }
 
+diff_cmd_help () {
+	echo "Use ECMerge (requires a graphical session)"
+}
+
 merge_cmd () {
 	if $base_present
 	then
@@ -12,3 +16,7 @@ merge_cmd () {
 			--default --mode=merge2 --to="$MERGED"
 	fi
 }
+
+merge_cmd_help () {
+	echo "Use ECMerge (requires a graphical session)"
+}
diff --git a/mergetools/emerge b/mergetools/emerge
index d1ce513ff5..fc6892cc95 100644
--- a/mergetools/emerge
+++ b/mergetools/emerge
@@ -2,6 +2,10 @@ diff_cmd () {
 	"$merge_tool_path" -f emerge-files-command "$LOCAL" "$REMOTE"
 }
 
+diff_cmd_help () {
+	echo "Use Emacs' Emerge"
+}
+
 merge_cmd () {
 	if $base_present
 	then
@@ -17,6 +21,10 @@ merge_cmd () {
 	fi
 }
 
+merge_cmd_help () {
+	echo "Use Emacs' Emerge"
+}
+
 translate_merge_tool_path() {
 	echo emacs
 }
diff --git a/mergetools/examdiff b/mergetools/examdiff
index e72b06fc4d..6f53ca9161 100644
--- a/mergetools/examdiff
+++ b/mergetools/examdiff
@@ -2,6 +2,10 @@ diff_cmd () {
 	"$merge_tool_path" "$LOCAL" "$REMOTE" -nh
 }
 
+diff_cmd_help () {
+	echo "Use ExamDiff Pro (requires a graphical session)"
+}
+
 merge_cmd () {
 	if $base_present
 	then
@@ -11,6 +15,10 @@ merge_cmd () {
 	fi
 }
 
+merge_cmd_help () {
+	echo "Use ExamDiff Pro (requires a graphical session)"
+}
+
 translate_merge_tool_path() {
 	mergetool_find_win32_cmd "ExamDiff.com" "ExamDiff Pro"
 }
diff --git a/mergetools/guiffy b/mergetools/guiffy
index 8b23a13c41..3ed07efd16 100644
--- a/mergetools/guiffy
+++ b/mergetools/guiffy
@@ -2,6 +2,10 @@ diff_cmd () {
 	"$merge_tool_path" "$LOCAL" "$REMOTE"
 }
 
+diff_cmd_help () {
+	echo "Use Guiffy's Diff Tool (requires a graphical session)"
+}
+
 merge_cmd () {
 	if $base_present
 	then
@@ -13,6 +17,10 @@ merge_cmd () {
 	fi
 }
 
+merge_cmd_help () {
+	echo "Use Guiffy's Diff Tool (requires a graphical session)"
+}
+
 exit_code_trustable () {
 	true
 }
diff --git a/mergetools/kdiff3 b/mergetools/kdiff3
index 520cb914a1..ee8b3a0570 100644
--- a/mergetools/kdiff3
+++ b/mergetools/kdiff3
@@ -4,6 +4,10 @@ diff_cmd () {
 		"$LOCAL" "$REMOTE" >/dev/null 2>&1
 }
 
+diff_cmd_help () {
+	echo "Use KDiff3 (requires a graphical session)"
+}
+
 merge_cmd () {
 	if $base_present
 	then
@@ -22,6 +26,10 @@ merge_cmd () {
 	fi
 }
 
+merge_cmd_help () {
+	echo "Use KDiff3 (requires a graphical session)"
+}
+
 exit_code_trustable () {
 	true
 }
diff --git a/mergetools/kompare b/mergetools/kompare
index e8c0bfa678..4ce23dbe8b 100644
--- a/mergetools/kompare
+++ b/mergetools/kompare
@@ -2,10 +2,18 @@ can_merge () {
 	return 1
 }
 
+diff_cmd_help () {
+	echo "Use Kompare (requires a graphical session)"
+}
+
 diff_cmd () {
 	"$merge_tool_path" "$LOCAL" "$REMOTE"
 }
 
+merge_cmd_help () {
+	echo "Use Kompare (requires a graphical session)"
+}
+
 exit_code_trustable () {
 	true
 }
diff --git a/mergetools/meld b/mergetools/meld
index aab4ebb935..8ec0867e03 100644
--- a/mergetools/meld
+++ b/mergetools/meld
@@ -2,6 +2,10 @@ diff_cmd () {
 	"$merge_tool_path" "$LOCAL" "$REMOTE"
 }
 
+diff_cmd_help () {
+	echo "Use Meld (requires a graphical session)"
+}
+
 merge_cmd () {
 	check_meld_for_features
 
@@ -20,6 +24,10 @@ merge_cmd () {
 	fi
 }
 
+merge_cmd_help () {
+	echo "Use Meld (requires a graphical session) with optional \`auto merge\` (see \`git help mergetool\`'s \`CONFIGURATION\` section)"
+}
+
 # Get meld help message
 init_meld_help_msg () {
 	if test -z "$meld_help_msg"
diff --git a/mergetools/opendiff b/mergetools/opendiff
index b608dd6de3..44adf8f951 100644
--- a/mergetools/opendiff
+++ b/mergetools/opendiff
@@ -2,6 +2,10 @@ diff_cmd () {
 	"$merge_tool_path" "$LOCAL" "$REMOTE" | cat
 }
 
+diff_cmd_help () {
+	echo "Use FileMerge (requires a graphical session)"
+}
+
 merge_cmd () {
 	if $base_present
 	then
@@ -12,3 +16,7 @@ merge_cmd () {
 			-merge "$MERGED" | cat
 	fi
 }
+
+merge_cmd_help () {
+	echo "Use FileMerge (requires a graphical session)"
+}
diff --git a/mergetools/p4merge b/mergetools/p4merge
index 7a5b291dd2..f3cb197e58 100644
--- a/mergetools/p4merge
+++ b/mergetools/p4merge
@@ -19,6 +19,10 @@ diff_cmd () {
 	fi
 }
 
+diff_cmd_help () {
+	echo "Use HelixCore P4Merge (requires a graphical session)"
+}
+
 merge_cmd () {
 	if ! $base_present
 	then
@@ -34,3 +38,7 @@ create_empty_file () {
 
 	printf "%s" "$empty_file"
 }
+
+merge_cmd_help () {
+	echo "Use HelixCore P4Merge (requires a graphical session)"
+}
diff --git a/mergetools/smerge b/mergetools/smerge
index 9c2e6f6fd7..5410835a6b 100644
--- a/mergetools/smerge
+++ b/mergetools/smerge
@@ -2,6 +2,10 @@ diff_cmd () {
 	"$merge_tool_path" mergetool "$LOCAL" "$REMOTE" -o "$MERGED"
 }
 
+diff_cmd_help () {
+	echo "Use Sublime Merge (requires a graphical session)"
+}
+
 merge_cmd () {
 	if $base_present
 	then
@@ -10,3 +14,7 @@ merge_cmd () {
 		"$merge_tool_path" mergetool "$LOCAL" "$REMOTE" -o "$MERGED"
 	fi
 }
+
+merge_cmd_help () {
+	echo "Use Sublime Merge (requires a graphical session)"
+}
diff --git a/mergetools/tkdiff b/mergetools/tkdiff
index eee5cb57e3..66906a720d 100644
--- a/mergetools/tkdiff
+++ b/mergetools/tkdiff
@@ -2,6 +2,10 @@ diff_cmd () {
 	"$merge_tool_path" "$LOCAL" "$REMOTE"
 }
 
+diff_cmd_help () {
+	echo "Use TkDiff (requires a graphical session)"
+}
+
 merge_cmd () {
 	if $base_present
 	then
@@ -14,3 +18,7 @@ merge_cmd () {
 exit_code_trustable () {
 	true
 }
+
+merge_cmd_help () {
+	echo "Use TkDiff (requires a graphical session)"
+}
diff --git a/mergetools/tortoisemerge b/mergetools/tortoisemerge
index d7ab666a59..507edcd444 100644
--- a/mergetools/tortoisemerge
+++ b/mergetools/tortoisemerge
@@ -2,6 +2,10 @@ can_diff () {
 	return 1
 }
 
+diff_cmd_help () {
+	echo "Use TortoiseMerge (requires a graphical session)"
+}
+
 merge_cmd () {
 	if $base_present
 	then
@@ -30,3 +34,7 @@ translate_merge_tool_path() {
 		echo tortoisemerge
 	fi
 }
+
+merge_cmd_help () {
+	echo "Use TortoiseMerge (requires a graphical session)"
+}
diff --git a/mergetools/vimdiff b/mergetools/vimdiff
index 96f6209a04..461a89b6f9 100644
--- a/mergetools/vimdiff
+++ b/mergetools/vimdiff
@@ -1,49 +1,495 @@
+# This script can be run in two different contexts:
+#
+#   - From git, when the user invokes the "vimdiff" merge tool. In this context
+#     this script expects the following environment variables (among others) to
+#     be defined (which is something "git" takes care of):
+#
+#       - $BASE
+#       - $LOCAL
+#       - $REMOTE
+#       - $MERGED
+#
+#     In this mode, all this script does is to run the next command:
+#
+#         vim -f -c ... $LOCAL $BASE $REMOTE $MERGED
+#
+#     ...where the "..." string depends on the value of the
+#     "mergetool.vimdiff.layout" configuration variable and is used to open vim
+#     with a certain layout of buffers, windows and tabs.
+#
+#   - From a script inside the unit tests framework folder ("t" folder) by
+#     sourcing this script and then manually calling "run_unit_tests", which
+#     will run a battery of unit tests to make sure nothing breaks.
+#     In this context this script does not expect any particular environment
+#     variable to be set.
+
+
+################################################################################
+## Internal functions (not meant to be used outside this script)
+################################################################################
+
+debug_print () {
+	# Send message to stderr if global variable GIT_MERGETOOL_VIMDIFF is set
+	# to "true"
+
+	if test -n "$GIT_MERGETOOL_VIMDIFF_DEBUG"
+	then
+		>&2 echo "$@"
+	fi
+}
+
+substring () {
+	# Return a substring of $1 containing $3 characters starting at
+	# zero-based offset $2.
+	#
+	# Examples:
+	#
+	#   substring "Hello world" 0 4  --> "Hell"
+	#   substring "Hello world" 3 4  --> "lo w"
+	#   substring "Hello world" 3 10 --> "lo world"
+
+	STRING=$1
+	START=$2
+	LEN=$3
+
+	echo "$STRING" | cut -c$(( START + 1 ))-$(( START + $LEN ))
+}
+
+gen_cmd_aux () {
+	# Auxiliary function used from "gen_cmd()".
+	# Read that other function documentation for more details.
+
+	LAYOUT=$1
+	CMD=$2  # This is a second (hidden) argument used for recursion
+
+	debug_print
+	debug_print "LAYOUT    : $LAYOUT"
+	debug_print "CMD       : $CMD"
+
+	if test -z "$CMD"
+	then
+		CMD="echo" # vim "nop" operator
+	fi
+
+	start=0
+	end=${#LAYOUT}
+
+	nested=0
+	nested_min=100
+
+
+	# Step 1:
+	#
+	# Increase/decrease "start"/"end" indices respectively to get rid of
+	# outer parenthesis.
+	#
+	# Example:
+	#
+	#   - BEFORE: (( LOCAL , BASE ) / MERGED )
+	#   - AFTER :  ( LOCAL , BASE ) / MERGED
+
+	oldIFS=$IFS
+	IFS=#
+	for c in $(echo "$LAYOUT" | sed 's:.:&#:g')
+	do
+		if test "$c" = " "
+		then
+			continue
+		fi
+
+		if test "$c" = "("
+		then
+			nested=$(( nested + 1 ))
+			continue
+		fi
+
+		if test "$c" = ")"
+		then
+			nested=$(( nested - 1 ))
+			continue
+		fi
+
+		if test "$nested" -lt "$nested_min"
+		then
+			nested_min=$nested
+		fi
+	done
+	IFS=$oldIFS
+
+	debug_print "NESTED MIN: $nested_min"
+
+	while test "$nested_min" -gt "0"
+	do
+		start=$(( start + 1 ))
+		end=$(( end - 1 ))
+
+		start_minus_one=$(( start - 1 ))
+
+		while ! test "$(substring "$LAYOUT" "$start_minus_one" 1)" = "("
+		do
+			start=$(( start + 1 ))
+			start_minus_one=$(( start_minus_one + 1 ))
+		done
+
+		while ! test "$(substring "$LAYOUT" "$end" 1)" = ")"
+		do
+			end=$(( end - 1 ))
+		done
+
+		nested_min=$(( nested_min - 1 ))
+	done
+
+	debug_print "CLEAN     : $(substring "$LAYOUT" "$start" "$(( end - start ))")"
+
+
+	# Step 2:
+	#
+	# Search for all valid separators ("+", "/" or ",") which are *not*
+	# inside parenthesis. Save the index at which each of them makes the
+	# first appearance.
+
+	index_new_tab=""
+	index_horizontal_split=""
+	index_vertical_split=""
+
+	nested=0
+	i=$(( start - 1 ))
+
+	oldIFS=$IFS
+	IFS=#
+	for c in $(substring "$LAYOUT" "$start" "$(( end - start ))" | sed 's:.:&#:g');
+	do
+		i=$(( i + 1 ))
+
+		if test "$c" = " "
+		then
+			continue
+		fi
+
+		if test "$c" = "("
+		then
+			nested=$(( nested + 1 ))
+			continue
+		fi
+
+		if test "$c" = ")"
+		then
+			nested=$(( nested - 1 ))
+			continue
+		fi
+
+		if test "$nested" = 0
+		then
+			current=$c
+
+			if test "$current" = "+"
+			then
+				if test -z "$index_new_tab"
+				then
+					index_new_tab=$i
+				fi
+
+			elif test "$current" = "/"
+			then
+				if test -z "$index_horizontal_split"
+				then
+					index_horizontal_split=$i
+				fi
+
+			elif test "$current" = ","
+			then
+				if test -z "$index_vertical_split"
+				then
+					index_vertical_split=$i
+				fi
+			fi
+		fi
+	done
+	IFS=$oldIFS
+
+
+	# Step 3:
+	#
+	# Process the separator with the highest order of precedence
+	# (";" has the highest precedence and "|" the lowest one).
+	#
+	# By "process" I mean recursively call this function twice: the first
+	# one with the substring at the left of the separator and the second one
+	# with the one at its right.
+
+	terminate="false"
+
+	if ! test -z "$index_new_tab"
+	then
+		before="-tabnew"
+		after="tabnext"
+		index=$index_new_tab
+		terminate="true"
+
+	elif ! test -z "$index_horizontal_split"
+	then
+		before="split"
+		after="wincmd j"
+		index=$index_horizontal_split
+		terminate="true"
+
+	elif ! test -z "$index_vertical_split"
+	then
+		before="vertical split"
+		after="wincmd l"
+		index=$index_vertical_split
+		terminate="true"
+	fi
+
+	if  test "$terminate" = "true"
+	then
+		CMD="$CMD | $before"
+		CMD=$(gen_cmd_aux "$(substring "$LAYOUT" "$start" "$(( index - start ))")" "$CMD")
+		CMD="$CMD | $after"
+		CMD=$(gen_cmd_aux "$(substring "$LAYOUT" "$(( index + 1 ))" "$(( ${#LAYOUT} - index ))")" "$CMD")
+		echo "$CMD"
+		return
+	fi
+
+
+	# Step 4:
+	#
+	# If we reach this point, it means there are no separators and we just
+	# need to print the command to display the specified buffer
+
+	target=$(substring "$LAYOUT" "$start" "$(( end - start ))" | sed 's:[ @();|-]::g')
+
+	if test "$target" = "LOCAL"
+	then
+		CMD="$CMD | 1b"
+
+	elif test "$target" = "BASE"
+	then
+		CMD="$CMD | 2b"
+
+	elif test "$target" = "REMOTE"
+	then
+		CMD="$CMD | 3b"
+
+	elif test "$target" = "MERGED"
+	then
+		CMD="$CMD | 4b"
+
+	else
+		CMD="$CMD | ERROR: >$target<"
+	fi
+
+	echo "$CMD"
+	return
+}
+
+
+gen_cmd () {
+	# This function returns (in global variable FINAL_CMD) the string that
+	# you can use when invoking "vim" (as shown next) to obtain a given
+	# layout:
+	#
+	#   $ vim -f $FINAL_CMD "$LOCAL" "$BASE" "$REMOTE" "$MERGED"
+	#
+	# It takes one single argument: a string containing the desired layout
+	# definition.
+	#
+	# The syntax of the "layout definitions" is explained in "Documentation/
+	# mergetools/vimdiff.txt" but you can already intuitively understand how
+	# it works by knowing that...
+	#
+	#   * "+" means "a new vim tab"
+	#   * "/" means "a new vim horizontal split"
+	#   * "," means "a new vim vertical split"
+	#
+	# It also returns (in global variable FINAL_TARGET) the name ("LOCAL",
+	# "BASE", "REMOTE" or "MERGED") of the file that is marked with an "@",
+	# or "MERGED" if none of them is.
+	#
+	# Example:
+	#
+	#     gen_cmd "@LOCAL , REMOTE"
+	#     |
+	#     `-> FINAL_CMD    == "-c \"echo | vertical split | 1b | wincmd l | 3b | tabdo windo diffthis\" -c \"tabfirst\""
+	#         FINAL_TARGET == "LOCAL"
+
+	LAYOUT=$1
+
+
+	# Search for a "@" in one of the files identifiers ("LOCAL", "BASE",
+	# "REMOTE", "MERGED"). If not found, use "MERGE" as the default file
+	# where changes will be saved.
+
+	if echo "$LAYOUT" | grep @LOCAL >/dev/null
+	then
+		FINAL_TARGET="LOCAL"
+	elif echo "$LAYOUT" | grep @BASE >/dev/null
+	then
+		FINAL_TARGET="BASE"
+	else
+		FINAL_TARGET="MERGED"
+	fi
+
+
+	# Obtain the first part of vim "-c" option to obtain the desired layout
+
+	CMD=$(gen_cmd_aux "$LAYOUT")
+
+
+	# Adjust the just obtained script depending on whether more than one
+	# windows are visible or not
+
+	if echo "$LAYOUT" | grep ",\|/" >/dev/null
+	then
+		CMD="$CMD | tabdo windo diffthis"
+	else
+		CMD="$CMD | bufdo diffthis"
+	fi
+
+
+	# Add an extra "-c" option to move to the first tab (notice that we
+	# can't simply append the command to the previous "-c" string as
+	# explained here: https://github.com/vim/vim/issues/9076
+
+	FINAL_CMD="-c \"$CMD\" -c \"tabfirst\""
+}
+
+
+################################################################################
+## API functions (called from "git-mergetool--lib.sh")
+################################################################################
+
 diff_cmd () {
 	"$merge_tool_path" -R -f -d \
 		-c 'wincmd l' -c 'cd $GIT_PREFIX' "$LOCAL" "$REMOTE"
 }
 
+
+diff_cmd_help () {
+	TOOL=$1
+
+	case "$TOOL" in
+	nvimdiff*)
+		printf "Use Neovim"
+		;;
+	gvimdiff*)
+		printf "Use gVim (requires a graphical session)"
+		;;
+	vimdiff*)
+		printf "Use Vim"
+		;;
+	esac
+
+	return 0
+}
+
+
 merge_cmd () {
+	layout=$(git config mergetool.vimdiff.layout)
+
 	case "$1" in
 	*vimdiff)
-		if $base_present
+		if test -z "$layout"
 		then
-			"$merge_tool_path" -f -d -c '4wincmd w | wincmd J' \
-				"$LOCAL" "$BASE" "$REMOTE" "$MERGED"
-		else
-			"$merge_tool_path" -f -d -c 'wincmd l' \
-				"$LOCAL" "$MERGED" "$REMOTE"
+			# Default layout when none is specified
+			layout="(LOCAL,BASE,REMOTE)/MERGED"
 		fi
 		;;
 	*vimdiff1)
-		"$merge_tool_path" -f -d \
-			-c 'echon "Resolve conflicts leftward then save. Use :cq to abort."' \
-			"$LOCAL" "$REMOTE"
-		ret="$?"
-		if test "$ret" -eq 0
-		then
-			cp -- "$LOCAL" "$MERGED"
-		fi
-		return "$ret"
+		layout="@LOCAL,REMOTE"
 		;;
 	*vimdiff2)
-		"$merge_tool_path" -f -d -c 'wincmd l' \
-			"$LOCAL" "$MERGED" "$REMOTE"
+		layout="LOCAL,MERGED,REMOTE"
 		;;
 	*vimdiff3)
-		if $base_present
+		layout="MERGED"
+		;;
+	esac
+
+	gen_cmd "$layout"
+
+	debug_print ""
+	debug_print "FINAL CMD : $FINAL_CMD"
+	debug_print "FINAL TAR : $FINAL_TARGET"
+
+	if $base_present
+	then
+		eval "$merge_tool_path" \
+			-f "$FINAL_CMD" "$LOCAL" "$BASE" "$REMOTE" "$MERGED"
+	else
+		# If there is no BASE (example: a merge conflict in a new file
+		# with the same name created in both braches which didn't exist
+		# before), close all BASE windows using vim's "quit" command
+
+		FINAL_CMD=$(echo "$FINAL_CMD" | \
+			sed -e 's:2b:quit:g' -e 's:3b:2b:g' -e 's:4b:3b:g')
+
+		eval "$merge_tool_path" \
+			-f "$FINAL_CMD" "$LOCAL" "$REMOTE" "$MERGED"
+	fi
+
+	ret="$?"
+
+	if test "$ret" -eq 0
+	then
+		case "$FINAL_TARGET" in
+		LOCAL)
+			source_path="$LOCAL"
+			;;
+		REMOTE)
+			source_path="$REMOTE"
+			;;
+		MERGED|*)
+			# Do nothing
+			source_path=
+			;;
+		esac
+
+		if test -n "$source_path"
 		then
-			"$merge_tool_path" -f -d -c 'hid | hid | hid' \
-				"$LOCAL" "$REMOTE" "$BASE" "$MERGED"
-		else
-			"$merge_tool_path" -f -d -c 'hid | hid' \
-				"$LOCAL" "$REMOTE" "$MERGED"
+			cp "$source_path" "$MERGED"
 		fi
+	fi
+
+	return "$ret"
+}
+
+
+merge_cmd_help () {
+	TOOL=$1
+
+	case "$TOOL" in
+	nvimdiff*)
+		printf "Use Neovim "
+		;;
+	gvimdiff*)
+		printf "Use gVim (requires a graphical session) "
+		;;
+	vimdiff*)
+		printf "Use Vim "
+		;;
+	esac
+
+	case "$TOOL" in
+	*1)
+		echo "with a 2 panes layout (LOCAL and REMOTE)"
+		;;
+	*2)
+		echo "with a 3 panes layout (LOCAL, MERGED and REMOTE)"
+		;;
+	*3)
+		echo "where only the MERGED file is shown"
+		;;
+	*)
+		echo "with a custom layout (see \`git help mergetool\`'s \`BACKEND SPECIFIC HINTS\` section)"
 		;;
 	esac
+
+	return 0
 }
 
-translate_merge_tool_path() {
+
+translate_merge_tool_path () {
 	case "$1" in
 	nvimdiff*)
 		echo nvim
@@ -57,14 +503,121 @@ translate_merge_tool_path() {
 	esac
 }
 
+
 exit_code_trustable () {
 	true
 }
 
+
 list_tool_variants () {
-	for prefix in '' g n; do
-		for suffix in '' 1 2 3; do
-			echo "${prefix}vimdiff${suffix}"
+	if test "$TOOL_MODE" = "diff"
+	then
+		for prefix in '' g n
+		do
+			echo "${prefix}vimdiff"
+		done
+	else
+		for prefix in '' g n
+		do
+			for suffix in '' 1 2 3
+			do
+				echo "${prefix}vimdiff${suffix}"
+			done
 		done
+	fi
+}
+
+
+################################################################################
+## Unit tests (called from scripts inside the "t" folder)
+################################################################################
+
+run_unit_tests () {
+	# Function to make sure that we don't break anything when modifying this
+	# script.
+
+	NUMBER_OF_TEST_CASES=16
+
+	TEST_CASE_01="(LOCAL,BASE,REMOTE)/MERGED"   # default behaviour
+	TEST_CASE_02="@LOCAL,REMOTE"                # when using vimdiff1
+	TEST_CASE_03="LOCAL,MERGED,REMOTE"          # when using vimdiff2
+	TEST_CASE_04="MERGED"                       # when using vimdiff3
+	TEST_CASE_05="LOCAL/MERGED/REMOTE"
+	TEST_CASE_06="(LOCAL/REMOTE),MERGED"
+	TEST_CASE_07="MERGED,(LOCAL/REMOTE)"
+	TEST_CASE_08="(LOCAL,REMOTE)/MERGED"
+	TEST_CASE_09="MERGED/(LOCAL,REMOTE)"
+	TEST_CASE_10="(LOCAL/BASE/REMOTE),MERGED"
+	TEST_CASE_11="(LOCAL,BASE,REMOTE)/MERGED+BASE,LOCAL+BASE,REMOTE+(LOCAL/BASE/REMOTE),MERGED"
+	TEST_CASE_12="((LOCAL,REMOTE)/BASE),MERGED"
+	TEST_CASE_13="((LOCAL,REMOTE)/BASE),((LOCAL/REMOTE),MERGED)"
+	TEST_CASE_14="BASE,REMOTE+BASE,LOCAL"
+	TEST_CASE_15="  ((  (LOCAL , BASE , REMOTE) / MERGED))   +(BASE)   , LOCAL+ BASE , REMOTE+ (((LOCAL / BASE / REMOTE)) ,    MERGED   )  "
+	TEST_CASE_16="LOCAL,BASE,REMOTE / MERGED + BASE,LOCAL + BASE,REMOTE + (LOCAL / BASE / REMOTE),MERGED"
+
+	EXPECTED_CMD_01="-c \"echo | split | vertical split | 1b | wincmd l | vertical split | 2b | wincmd l | 3b | wincmd j | 4b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_02="-c \"echo | vertical split | 1b | wincmd l | 3b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_03="-c \"echo | vertical split | 1b | wincmd l | vertical split | 4b | wincmd l | 3b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_04="-c \"echo | 4b | bufdo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_05="-c \"echo | split | 1b | wincmd j | split | 4b | wincmd j | 3b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_06="-c \"echo | vertical split | split | 1b | wincmd j | 3b | wincmd l | 4b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_07="-c \"echo | vertical split | 4b | wincmd l | split | 1b | wincmd j | 3b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_08="-c \"echo | split | vertical split | 1b | wincmd l | 3b | wincmd j | 4b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_09="-c \"echo | split | 4b | wincmd j | vertical split | 1b | wincmd l | 3b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_10="-c \"echo | vertical split | split | 1b | wincmd j | split | 2b | wincmd j | 3b | wincmd l | 4b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_11="-c \"echo | -tabnew | split | vertical split | 1b | wincmd l | vertical split | 2b | wincmd l | 3b | wincmd j | 4b | tabnext | -tabnew | vertical split | 2b | wincmd l | 1b | tabnext | -tabnew | vertical split | 2b | wincmd l | 3b | tabnext | vertical split | split | 1b | wincmd j | split | 2b | wincmd j | 3b | wincmd l | 4b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_12="-c \"echo | vertical split | split | vertical split | 1b | wincmd l | 3b | wincmd j | 2b | wincmd l | 4b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_13="-c \"echo | vertical split | split | vertical split | 1b | wincmd l | 3b | wincmd j | 2b | wincmd l | vertical split | split | 1b | wincmd j | 3b | wincmd l | 4b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_14="-c \"echo | -tabnew | vertical split | 2b | wincmd l | 3b | tabnext | vertical split | 2b | wincmd l | 1b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_15="-c \"echo | -tabnew | split | vertical split | 1b | wincmd l | vertical split | 2b | wincmd l | 3b | wincmd j | 4b | tabnext | -tabnew | vertical split | 2b | wincmd l | 1b | tabnext | -tabnew | vertical split | 2b | wincmd l | 3b | tabnext | vertical split | split | 1b | wincmd j | split | 2b | wincmd j | 3b | wincmd l | 4b | tabdo windo diffthis\" -c \"tabfirst\""
+	EXPECTED_CMD_16="-c \"echo | -tabnew | split | vertical split | 1b | wincmd l | vertical split | 2b | wincmd l | 3b | wincmd j | 4b | tabnext | -tabnew | vertical split | 2b | wincmd l | 1b | tabnext | -tabnew | vertical split | 2b | wincmd l | 3b | tabnext | vertical split | split | 1b | wincmd j | split | 2b | wincmd j | 3b | wincmd l | 4b | tabdo windo diffthis\" -c \"tabfirst\""
+
+	EXPECTED_TARGET_01="MERGED"
+	EXPECTED_TARGET_02="LOCAL"
+	EXPECTED_TARGET_03="MERGED"
+	EXPECTED_TARGET_04="MERGED"
+	EXPECTED_TARGET_05="MERGED"
+	EXPECTED_TARGET_06="MERGED"
+	EXPECTED_TARGET_07="MERGED"
+	EXPECTED_TARGET_08="MERGED"
+	EXPECTED_TARGET_09="MERGED"
+	EXPECTED_TARGET_10="MERGED"
+	EXPECTED_TARGET_11="MERGED"
+	EXPECTED_TARGET_12="MERGED"
+	EXPECTED_TARGET_13="MERGED"
+	EXPECTED_TARGET_14="MERGED"
+	EXPECTED_TARGET_15="MERGED"
+	EXPECTED_TARGET_16="MERGED"
+
+	at_least_one_ko="false"
+
+	for i in $(seq -w 1 99)
+	do
+		if test "$i" -gt $NUMBER_OF_TEST_CASES
+		then
+			break
+		fi
+
+		gen_cmd "$(eval echo \${TEST_CASE_"$i"})"
+
+		if test "$FINAL_CMD" = "$(eval echo \${EXPECTED_CMD_"$i"})" \
+			&& test "$FINAL_TARGET" = "$(eval echo \${EXPECTED_TARGET_"$i"})"
+		then
+			printf "Test Case #%02d: OK\n" "$(echo "$i" | sed 's/^0*//')"
+		else
+			printf "Test Case #%02d: KO !!!!\n" "$(echo "$i" | sed 's/^0*//')"
+			echo "  FINAL_CMD              : $FINAL_CMD"
+			echo "  FINAL_CMD (expected)   : $(eval echo \${EXPECTED_CMD_"$i"})"
+			echo "  FINAL_TARGET           : $FINAL_TARGET"
+			echo "  FINAL_TARGET (expected): $(eval echo \${EXPECTED_TARGET_"$i"})"
+			at_least_one_ko="true"
+		fi
 	done
+
+	if test "$at_least_one_ko" = "true"
+	then
+		return 255
+	else
+		return 0
+	fi
 }
diff --git a/mergetools/winmerge b/mergetools/winmerge
index 74d03259fd..36c72dde6e 100644
--- a/mergetools/winmerge
+++ b/mergetools/winmerge
@@ -3,6 +3,10 @@ diff_cmd () {
 	return 0
 }
 
+diff_cmd_help () {
+	echo "Use WinMerge (requires a graphical session)"
+}
+
 merge_cmd () {
 	# mergetool.winmerge.trustExitCode is implicitly false.
 	# touch $BACKUP so that we can check_unchanged.
@@ -13,3 +17,7 @@ merge_cmd () {
 translate_merge_tool_path() {
 	mergetool_find_win32_cmd "WinMergeU.exe" "WinMerge"
 }
+
+merge_cmd_help () {
+	echo "Use WinMerge (requires a graphical session)"
+}
diff --git a/mergetools/xxdiff b/mergetools/xxdiff
index d5ce467995..cd205f9842 100644
--- a/mergetools/xxdiff
+++ b/mergetools/xxdiff
@@ -12,6 +12,10 @@ diff_cmd () {
 	fi
 }
 
+diff_cmd_help () {
+	echo "Use xxdiff (requires a graphical session)"
+}
+
 merge_cmd () {
 	if $base_present
 	then
@@ -28,3 +32,7 @@ merge_cmd () {
 			--merged-file "$MERGED" "$LOCAL" "$REMOTE"
 	fi
 }
+
+merge_cmd_help () {
+	echo "Use xxdiff (requires a graphical session)"
+}
diff --git a/t/t7609-mergetool--lib.sh b/t/t7609-mergetool--lib.sh
new file mode 100755
index 0000000000..d848fe6442
--- /dev/null
+++ b/t/t7609-mergetool--lib.sh
@@ -0,0 +1,14 @@
+#!/bin/sh
+
+test_description='git mergetool
+
+Testing basic merge tools options'
+
+. ./test-lib.sh
+
+test_expect_success 'mergetool --tool=vimdiff creates the expected layout' '
+	. $GIT_BUILD_DIR/mergetools/vimdiff &&
+	run_unit_tests
+'
+
+test_done