Make tests pass with recent git (1.7.0 and up).

It seems that in older versions, --message="" was interpreted as "use the
default commit message" instead of "use an empty commit message", and
git-subtree was depending on this behaviour.  Now we don't, so tests pass
again.

git-subtree.sh

@@ -603,7 +603,11 @@ cmd_merge()
 		debug "New squash commit: $new"
 		rev="$new"
 	fi
-	
+	
+	if [ -n "$message" ]; then
+		git merge -s subtree --message="$message" $rev
+	else
+		git merge -s subtree $rev
 	fi
 }