[1.2.X] Fixed #10918 -- Ensure that the search widget on a raw_id_admin uses...

[1.2.X] Fixed #10918 -- Ensure that the search widget on a raw_id_admin uses the right field name when the ForeignKey has a to_field definition. Thanks to David Cramer for the report, Collin Anderson for the fix, and Julien Phalip for the test.

Backport of r15657 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15659 bcc190cf-cafb-0310-a4f2-bffc1f526a37