Add a helper method to Image to parse ancestor string.

data/model/repository.py

@@ -157,7 +157,7 @@ def garbage_collect_repo(repo):
     def gen_referenced_ancestors():
       for tagged_image in tagged_images:
         # The ancestor list is in the format '/1/2/3/', extract just the ids
-        ancestor_id_strings = tagged_image.ancestors.split('/')[1:-1]
+        ancestor_id_strings = tagged_image.ancestor_list()
         for img_id_str in ancestor_id_strings:
           yield int(img_id_str)
         yield tagged_image.id