cache-tree: use is_empty_tree_oid

When comparing an object ID against that of the empty tree, use the
is_empty_tree_oid function to ensure that we abstract over the hash
algorithm properly.  In addition, this is more readable than a plain
oidcmp.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache-tree.c

@@ -385,7 +385,7 @@ static int update_one(struct cache_tree *it,
 		/*
 		 * "sub" can be an empty tree if all subentries are i-t-a.
 		 */
-		if (contains_ita && !oidcmp(oid, &empty_tree_oid))
+		if (contains_ita && is_empty_tree_oid(oid))
 			continue;
 
 		strbuf_grow(&buffer, entlen + 100);