optimize get_objects_for_user/group if klass is a queryset
optimize get_objects_for_user/group if klass is a queryset

In this case we want to reduce the set of objects to those given in
the queryset, for use-cases with lots of objects with explicit
permissions.

See https://github.com/django-guardian/django-guardian/issues/189

lukaszb/django-guardian,lukaszb/django-guardian,lukaszb/django-guardian