git-merge: make it usable as the first class UI

git-merge: make it usable as the first class UI

This teaches the oft-requested syntax

	git merge $commit

to implement merging the named commit to the current branch.
This hopefully would make "git merge" usable as the first class
UI instead of being a mere backend for "git pull".

Most notably, $commit above can be any committish, so you can
say for example:

	git merge js/shortlog~2

to merge early part of a topic branch without merging the rest
of it.

A custom merge message can be given with the new --message=<msg>
parameter.  The message is prepended in front of the usual
"Merge ..." message autogenerated with fmt-merge-message.

Signed-off-by: Junio C Hamano <junkio@cox.net>