Verify that the object type matches for tree/commit objects even before parsing.

The type doesn't come from the parsing, the type also has to match the usage.

tree.c

@@ -80,7 +80,7 @@ struct tree *lookup_tree(unsigned char *sha1)
 		ret->object.type = tree_type;
 		return ret;
 	}
-	if (obj->parsed && obj->type != tree_type) {
+	if (obj->type != tree_type) {
 		error("Object %s is a %s, not a tree", 
 		      sha1_to_hex(sha1), obj->type);
 		return NULL;