Allow for matches against unsaved objects in querysets (which will therefore

match nothing). This allows for some more straightforward code in the admin
interface.

Fixed #7488 (all the debugging there was done by Brian Rosner, who narrowed it
down to the item in this patch).


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8061 bcc190cf-cafb-0310-a4f2-bffc1f526a37