Fixed #13227 -- Ensure that the query cache is flushed when a QuerySet is deepcopied, avoiding problems when an evaluated queryset is used as a subquery. Thanks to claudep for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12970 bcc190cf-cafb-0310-a4f2-bffc1f526a37