Renamed 'reversed' variable to 'is_reversed' in ForLoop template tag...

Renamed 'reversed' variable to 'is_reversed' in ForLoop template tag implementation, to avoid potential conflicts with the 'reversed' built-in and to make the code more obvious

git-svn-id: http://code.djangoproject.com/svn/django/trunk@6976 bcc190cf-cafb-0310-a4f2-bffc1f526a37