Allow git-merge not to commit.

Allow git-merge not to commit.

Martin Langhoff wants to use git-merge from outside git-pull and wants
to do further processing; for this, he wants git-merge no to commit
even when it cleanly merges.  I think other script writers would want
something like that as well, so here it is.

Instead of the "merge commit message" parameter (which usually is made
for you by "git-pull" which calls this command), you pass an empty
string to it.  Then it will not update your HEAD -- you can do whatever
you want with the resulting index file, which contains the merge results.

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