Fixed #17530 - Fixture loading with deferred constraints broken for Postgres...

Fixed #17530 - Fixture loading with deferred constraints broken for Postgres for fixtures from multiple dirs/files

Thanks to claudep for the review.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17372 bcc190cf-cafb-0310-a4f2-bffc1f526a37