across inherited models.
Completely by accident, this also allows values() queries to include fields
from related models, providing it is crossing a single-valued relation
(one-to-one, many-to-one). Many-to-many values() fields still aren't supported,
since that requires actual thinking. So this refs #5768.
git-svn-id: http://code.djangoproject.com/svn/django/branches/queryset-refactor@7230 bcc190cf-cafb-0310-a4f2-bffc1f526a37