Fixed #12510. Changed ModelChoiceField to stop using some of its superclasses...

Fixed #12510. Changed ModelChoiceField to stop using some of its superclasses implementation. This could cause more than one query when generating choices. Thanks, Petr Marhoun and Honza Kral.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12211 bcc190cf-cafb-0310-a4f2-bffc1f526a37