Merge branch 'ad/commit-have-m-option' into maint

"git commit" misbehaved in a few minor ways when an empty message
is given via -m '', all of which has been corrected.

* ad/commit-have-m-option:
commit: do not ignore an empty message given by -m ''
commit: --amend -m '' silently fails to wipe message

diff --cc builtin/commit.c
Simple merge