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>