[1.2.X] Fixed #14102 -- Ensure that fields that have been excluded from a form...

[1.2.X] Fixed #14102 -- Ensure that fields that have been excluded from a form aren't included in the unique_for_* checks, either. Thanks to Travis Cline for the report and fix.

Backport of r13598 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13599 bcc190cf-cafb-0310-a4f2-bffc1f526a37