Fixed bug with QuerySet.exclude() failing to do negations on Q objects, and

at the same time generalised exclude/QNot so that they work for 'external'
Q objects i.e. ones that simply have 'get_sql' defined.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@2997 bcc190cf-cafb-0310-a4f2-bffc1f526a37