merge-tree: add a new --quiet flag

Git Forges may be interested in whether two branches can be merged while
not being interested in what the resulting merge tree is nor which files
conflicted.  For such cases, add a new --quiet flag which
will make use of the new mergeability_only flag added to merge-ort in
the previous commit.  This option allows the merge machinery to, in the
outer layer of the merge:
    * exit early when a conflict is detected
    * avoid writing (most) merged blobs/trees to the object store

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 38 insertions, 0 deletions

diff --git a/t/t4301-merge-tree-write-tree.sh b/t/t4301-merge-tree-write-tree.sh
index f9c5883a7f..6e117ee93c 100755
--- a/t/t4301-merge-tree-write-tree.sh
+++ b/t/t4301-merge-tree-write-tree.sh
@@ -54,6 +54,25 @@ test_expect_success setup '
 	git commit -m first-commit
 '
 
+test_expect_success '--quiet on clean merge' '
+	# Get rid of loose objects to start with
+	git gc &&
+	echo "0 objects, 0 kilobytes" >expect &&
+	git count-objects >actual &&
+	test_cmp expect actual &&
+
+	# Ensure merge is successful (exit code of 0)
+	git merge-tree --write-tree --quiet side1 side3 >output &&
+
+	# Ensure there is no output
+	test_must_be_empty output &&
+
+	# Ensure no loose objects written (all new objects written would have
+	# been in "outer layer" of the merge)
+	git count-objects >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'Clean merge' '
 	TREE_OID=$(git merge-tree --write-tree side1 side3) &&
 	q_to_tab <<-EOF >expect &&
@@ -72,6 +91,25 @@ test_expect_success 'Failed merge without rename detection' '
 	grep "CONFLICT (modify/delete): numbers deleted" out
 '
 
+test_expect_success  '--quiet on conflicted merge' '
+	# Get rid of loose objects to start with
+	git gc &&
+	echo "0 objects, 0 kilobytes" >expect &&
+	git count-objects >actual &&
+	test_cmp expect actual &&
+
+	# Ensure merge has conflict
+	test_expect_code 1 git merge-tree --write-tree --quiet side1 side2 >output &&
+
+	# Ensure there is no output
+	test_must_be_empty output &&
+
+	# Ensure no loose objects written (all new objects written would have
+	# been in "outer layer" of the merge)
+	git count-objects >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'Content merge and a few conflicts' '
 	git checkout side1^0 &&
 	test_must_fail git merge side2 &&