git-merge: squelch needless error message.

git-merge: squelch needless error message.

While deciding if the new style command line argument is a tag
or a branch, we checked it with "git show-ref -s --verify" to
see if results in an error, but when it is not a branch, the
check leaked the error message out, which was not needed to be
shown to the end user.

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