Make fsck reachability avoid doing unnecessary work for

parents that we reach multiple ways.

This doesn't matter right now. It _will_ matter once we have
complex revision graphs.

fsck-cache.c

@@ -108,6 +108,9 @@ static void mark_reachable(struct revision *rev)
 {
 	struct parent *p = rev->parent;
 
+	/* If we've been here already, don't bother */
+	if (rev->flags & REACHABLE)
+		return;
 	rev->flags |= REACHABLE | USED;
 	while (p) {
 		mark_reachable(p->parent);