Simplify merges even more aggressively.

If any one of the parents is the same as the current one, then clearly the
other parent branch isn't important, so throw it away entirely.

Can't remember why I didn't do this before, but if I rediscover it, it
definitely needs a unit test.

git-subtree.sh

@@ -286,7 +286,7 @@ copy_or_skip()
 		fi
 	done
 	
-	if [ -n "$identical" -a -z "$nonidentical" ]; then
+	if [ -n "$identical" ]; then
 		echo $identical
 	else
 		copy_commit $rev $tree "$p" || exit $?