[1.2.X] Fixed #15032 -- Replaced 1.2.X implementation of admin changelist...

[1.2.X] Fixed #15032 -- Replaced 1.2.X implementation of admin changelist filtering security fix (r15031/r15033) with the one from trunk so another valid filter usage scenario (using model inheritance) is still possible. Thanks rene for reporting this.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15177 bcc190cf-cafb-0310-a4f2-bffc1f526a37