Revert recent "git merge <msg> HEAD <commit>..." deprecation

Revert recent "git merge <msg> HEAD <commit>..." deprecation

This reverts commit c0ecb07048ce2123589a2f077d296e8cf29a9570 "git-pull.sh:
Fix call to git-merge for new command format" and

commit b81e00a965c62ca72a4b9db425ee173de147808d "git-merge: a deprecation
notice of the ancient command line syntax".

They caused a "git pull" (without any arguments, and without any local
commits---only to update to the other side) to warn that commit log
message is ignored because the merge resulted in a fast-forward.

Another possible solution is to add an extra option to "git merge" so that
"git pull" can tell it that the message given is not coming from the end
user (the canned message is passed just in case the merge resulted in a
non-ff and caused commit), but I think it is easier _not_ to deprecate the
old syntax.

Signed-off-by: Junio C Hamano <gitster@pobox.com>