Add image ancestry and update the test db accordingly.

data/database.py

@@ -89,6 +89,9 @@ class Image(BaseModel):
   comment = CharField(null=True)
   repository = ForeignKeyField(Repository)
 
+  # '/' separated list of ancestory ids, e.g. /1/2/6/7/10/
+  ancestors = CharField(index=True, default='/', max_length=65535)
+
   class Meta:
     database = db
     indexes = (