tree: simplify parse_tree_indirect()

Reduce code duplication by turning parse_tree_indirect() into a wrapper
of repo_peel_to_type().  This avoids a segfault when handling a broken
tag where ->tagged is NULL.  The new version also checks the return
value of parse_object() that was ignored by the old one.

Initial-patch-by: Stefan Sperling <stsp@stsp.name>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

tree.c

@@ -244,19 +244,7 @@ void free_tree_buffer(struct tree *tree)
 
 struct tree *parse_tree_indirect(const struct object_id *oid)
 {
-	struct object *obj = parse_object(the_repository, oid);
+	struct repository *r = the_repository;
-	do {
+	struct object *obj = parse_object(r, oid);
-		if (!obj)
+	return (struct tree *)repo_peel_to_type(r, NULL, 0, obj, OBJ_TREE);
-			return NULL;
-		if (obj->type == OBJ_TREE)
-			return (struct tree *) obj;
-		else if (obj->type == OBJ_COMMIT)
-			obj = &(get_commit_tree(((struct commit *)obj))->object);
-		else if (obj->type == OBJ_TAG)
-			obj = ((struct tag *) obj)->tagged;
-		else
-			return NULL;
-		if (!obj->parsed)
-			parse_object(the_repository, &obj->oid);
-	} while (1);
 }