Fixed #11956 -- Modified the handling of m2m relationships between subclasses....

Fixed #11956 -- Modified the handling of m2m relationships between subclasses. Thanks to nidi for the report, and astoneman for the suggestion on how to fix the problem.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12908 bcc190cf-cafb-0310-a4f2-bffc1f526a37