Merge branch 'ab/commit-m-with-fixup' into maint

"git commit --fixup" did not allow "-m<message>" option to be used
at the same time; allow it to annotate resulting commit with more
text.

* ab/commit-m-with-fixup:
commit: add support for --fixup <commit> -m"<extra message>"
commit doc: document that -c, -C, -F and --fixup with -m error

diff --cc builtin/commit.c
Simple merge

diff --cc t/t7500-commit.sh
Simple merge