[1.0.X] Fixed #9994 -- Fixed admin filtering when to_field is used on relations.

Thanks to jzylks for diagnosing this one.

Backport of r10139 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@10140 bcc190cf-cafb-0310-a4f2-bffc1f526a37