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

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>