This option makes it convenient to construct commit messages for use
with 'rebase --autosquash'. The resulting commit message will be
"fixup! ..." where "..." is the subject line of the specified commit
message.
Example usage:
$ git commit --fixup HEAD~2
Signed-off-by: Pat Notz <patnotz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>