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>

git-merge.sh

@@ -189,7 +189,7 @@ else
 	merge_name=$(for remote
 		do
 			rh=$(git-rev-parse --verify "$remote"^0 2>/dev/null) &&
-			bh=$(git show-ref -s --verify "refs/heads/$remote") &&
+			bh=$(git show-ref -s --verify "refs/heads/$remote" 2>/dev/null) &&
 			if test "$rh" = "$bh"
 			then
 				echo "$rh		branch '$remote' of ."