rebase -i: respect commit.cleanup when picking fixups

rebase -i: respect commit.cleanup when picking fixups

If the user uses a prepare-commit-msg hook to add comments to the
commit message template and sets commit.cleanup to remove them when the
commit is created then the comments will not be removed when rebase
commits the final command in a chain of "fixup" commands[1].  This
happens because f7d42ceec52 (rebase -i: do leave commit message intact
in fixup! chains, 2021-01-28) started passing the VERBATIM_MSG flag
when committing the final command in a chain of "fixup" commands. That
change was added in response to a bug report[2] where the commit
message was being cleaned up when it should not be. The cause of that
bug was that before f7d42ceec52 the sequencer passed CLEANUP_MSG
when committing the final fixup. That commit should have simply
removed the CLEANUP_MSG flag, not changed it to VERBATIM_MSG. Using
VERBATIM_MSG ignores the user's commit.cleanup config when committing
the final fixup which means it behaves differently to an ordinary
"pick" command which respects commit.cleanup.

Fix this by not setting an explicit cleanup flag when committing the
final fixup which matches the way "pick" commands behave. The test
added in f7d42ceec52 is replaced with one that checks that "fixup"
and "pick" commands do not clean up the message when commit.cleanup
is not set and do clean up the message when it is set.

[1] https://lore.kernel.org/git/CA+itcS3DxbgpFy2aPRvHQvTAYE=dU0kfeDdidVwWLU=rBAWR4w@mail.gmail.com
[2] https://lore.kernel.org/git/CANVGpwZGbzYLMeMze64e_OU9p3bjyEgzC5thmNBr6LttBt+YGw@mail.gmail.com

Reported-by: Simon Cheng <cyqsimon@gmail.com>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>