test-dump-cache-tree: invalid trees are not errors

Do not treat known-invalid trees as errors even when their subtree_nr is
incorrect.  Because git already knows that these trees are invalid,
an incorrect subtree_nr will not cause problems.

Add a couple of comments.

Signed-off-by: David Turner <dturner@twitter.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

test-dump-cache-tree.c

@@ -26,16 +26,16 @@ static int dump_cache_tree(struct cache_tree *it,
 		return 0;
 
 	if (it->entry_count < 0) {
+		/* invalid */
 		dump_one(it, pfx, "");
 		dump_one(ref, pfx, "#(ref) ");
-		if (it->subtree_nr != ref->subtree_nr)
-			errs = 1;
 	}
 	else {
 		dump_one(it, pfx, "");
 		if (hashcmp(it->sha1, ref->sha1) ||
 		    ref->entry_count != it->entry_count ||
 		    ref->subtree_nr != it->subtree_nr) {
+			/* claims to be valid but is lying */
 			dump_one(ref, pfx, "#(ref) ");
 			errs = 1;
 		}