Fixed #8115: avoid a infiniate loop when collecting related objects for deletion.

I can't reproduce the original error leading to #8115 and the patch. However, the only harm this change could cause is to raise more `CyclicDependency` exceptions than strictly necessary. That's better than infinite loops, at least, and it's easier to clean up in the future when we figure out the actual fix.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8807 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/models/query.py

@@ -95,6 +95,8 @@ class CollectedObjects(object):
         while len(dealt_with) < len(models):
             found = False
             for model in models:
+                if model in dealt_with:
+                    continue
                 children = self.children.setdefault(model, [])
                 if len([c for c in children if c not in dealt_with]) == 0:
                     dealt_with[model] = None