commit: add support for --fixup <commit> -m"<extra message>"

Add support for supplying the -m option with --fixup. Doing so has errored out ever since --fixup was introduced. Before this, the only way to amend the fixup message while committing was to use --edit and amend it in the editor. The use-case for this feature is one of: * Leaving a quick note to self when creating a --fixup commit when it's not self-evident why the commit should be squashed without a note into another one. * (Ab)using the --fixup feature to "fix up" commits that have already been pushed to a branch that doesn't allow non-fast-forwards, i.e. just noting "this should have been part of that other commit", and if the history ever got rewritten in the future the two should be combined. In such a case you might want to leave a small message, e.g. "forgot this part, which broke XYZ". With this, --fixup <commit> -m"More" -m"Details" will result in a commit message like: !fixup <subject of <commit>> More Details The reason the test being added here seems to squash "More" at the end of the subject line of the commit being fixed up is because the test code is using "%s%b" so the body immediately follows the subject, it's not a bug in this code, and other tests t7500-commit.sh do the same thing. When the --fixup option was initially added the "Option -m cannot be combined" error was expanded from -c, -C and -F to also include --fixup[1] Those options could also support combining with -m, but given what they do I can't think of a good use-case for doing that, so I have not made the more invasive change of splitting up the logic in commit.c to first act on those, and then on -m options. 1. d71b8ba7c9 ("commit: --fixup option for use with rebase --autosquash", 2010-11-02) Helped-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>