Optimised use of 'in' operator on QuerySet using an explicit __contains__ method.

Without this change, use of 'in' on a QuerySet resulted in ._result_cache
being fully populated, which sometimes is unnecessary work.



git-svn-id: http://code.djangoproject.com/svn/django/trunk@11803 bcc190cf-cafb-0310-a4f2-bffc1f526a37