Disable GC for repositories with manifest lists

This is a temporary measure so we can roll V2_2 out for internal customer testing without having to worry about GC interacting oddly with it

data/model/repository.py

@@ -13,7 +13,7 @@ from data.database import (
   RepositoryPermission, RepositoryActionCount, Role, RepositoryAuthorizedEmail,
   DerivedStorageForImage, Label, db_for_update, get_epoch_timestamp,
   db_random_func, db_concat_func, RepositorySearchScore, RepositoryKind, ApprTag,
-  ManifestLegacyImage, Manifest)
+  ManifestLegacyImage, Manifest, ManifestChild)
 from data.text import prefix_search
 from util.itertoolrecipes import take
 
@@ -207,6 +207,15 @@ def garbage_collect_repo(repo, is_purge=False):
       """
   logger.debug('Garbage collecting repository %s', repo.id)
 
+  # TODO(jschorr): Update GC for the new data model.
+  if not is_purge:
+    try:
+      # Skip any repos with manifest lists in them, for now.
+      ManifestChild.get(repository=repo)
+      return True
+    except ManifestChild.DoesNotExist:
+      pass
+
   if is_purge:
     tag.purge_all_tags(repo)
     images_for_tags_removed = {i.id for i in Image.select().where(Image.repository == repo)}