git-merge: tighten error checking.

If a branch name to be merged is misspelled, the command leaked error
messages from underlying plumbing commands, which were helpful only
to people who know how the command are implemented to diagnose the
breakage, but simply puzzling and unhelpful for the end users.

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

git-merge.sh

@@ -188,13 +188,13 @@ else
 	# in this loop.
 	merge_name=$(for remote
 		do
-			rh=$(git-rev-parse --verify "$remote"^0 2>/dev/null)
+			rh=$(git-rev-parse --verify "$remote"^0 2>/dev/null) &&
 			if git show-ref -q --verify "refs/heads/$remote"
 			then
 				what=branch
 			else
 				what=commit
-			fi
+			fi &&
 			echo "$rh		$what '$remote'"
 		done | git-fmt-merge-msg
 	)
@@ -209,7 +209,7 @@ test "$rloga" = '' && rloga="merge: $@"
 remoteheads=
 for remote
 do
-	remotehead=$(git-rev-parse --verify "$remote"^0) ||
+	remotehead=$(git-rev-parse --verify "$remote"^0 2>/dev/null) ||
 	    die "$remote - not something we can merge"
 	remoteheads="${remoteheads}$remotehead "
 done