t3701: add a test for the different `add -p` prompts

The `git add -p` command offers different prompts for regular diff hunks
vs mode change pseudo hunks vs diffs deleting files.

Let's cover this in the regresion test suite, in preparation for
re-implementing `git add -p` in C.

For the mode change prompt, we use a trick that lets this test case pass
even on systems without executable bit, i.e. where `core.filemode =
false` (such as Windows): we first add the file to the index with `git
add --chmod=+x`, and then call `git add -p` with `core.filemode` forced
to `true`. The file on disk has no executable bit set, therefore we will
see a mode change.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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