git-merge: fix confusion between tag and branch

git-merge: fix confusion between tag and branch

In a repository with core.warnambiguousrefs turned off, and with
a branch and a tag that have the same name 'frotz',

	git merge frotz

would merge the commit pointed at by the tag 'frotz' but
incorrectly would identify what was merged as 'branch frotz' in
the merge message.

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