[1.2.X] Fixed a security issue in the file session backend. Disclosure and...

[1.2.X] Fixed a security issue in the file session backend.  Disclosure and new release forthcoming.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15468 bcc190cf-cafb-0310-a4f2-bffc1f526a37