t7505: add tests for cherry-pick and rebase -i/-p

t7505: add tests for cherry-pick and rebase -i/-p

Check that cherry-pick and rebase call the 'prepare-commit-msg' hook
correctly. The expected values for the hook arguments are taken to
match the current master branch. I think there is scope for improving
the arguments passed so they make a bit more sense - for instance
cherry-pick currently passes different arguments depending on whether
the commit message is being edited. Also the arguments for rebase
could be improved. Commit 7c4188360ac ("rebase -i: proper
prepare-commit-msg hook argument when squashing", 2008-10-3) apparently
changed things so that when squashing rebase would pass 'squash' as
the argument to the hook but that has been lost.

I think that it would make more sense to pass 'message' for revert and
cherry-pick -x/-s (i.e. cases where there is a new message or the
current message in modified by the command), 'squash' when squashing
with a new message and 'commit HEAD/CHERRY_PICK_HEAD'
otherwise (picking and squashing without a new message).

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>