Made some types of nested update queries very slightly more efficient at the

database level. Also worked around the fact that MySQL (and maybe other
backends we don't know about) cannot select from the table they're updating.

Fixed #7095.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7496 bcc190cf-cafb-0310-a4f2-bffc1f526a37