Fixed #10271, #10281 -- Fixed the handling multiple inline models that share...

Fixed #10271, #10281 --  Fixed the handling multiple inline models that share a common base class and have the link to the inline parent on the base class. Includes modifications that allow the equivalent handling for GenericFields. Thanks to Idan Gazit, Antti Kaihola (akaihola), and Alex Gaynor for their work on this patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10017 bcc190cf-cafb-0310-a4f2-bffc1f526a37