[1.1.X] Fixed #11956 -- Modified the handling of m2m relationships between...

[1.1.X] 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.

Backport of r12908 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12909 bcc190cf-cafb-0310-a4f2-bffc1f526a37