Test coverage update in preparation for further work on "git add -i".
* js/add-i-a-bit-more-tests:
apply --allow-overlap: fix a corner case
git add -p: use non-zero exit code when the diff generation failed
t3701: verify that the diff.algorithm config setting is handled
t3701: verify the shown messages when nothing can be added
t3701: add a test for the different `add -p` prompts
t3701: avoid depending on the TTY prerequisite
t3701: add a test for advanced split-hunk editing