Changed the row count value returned from update queries in some cases.

If an update only affected an ancestor model (not the child), we were
returning 0 for the number of rows updated. This could have been
misleading if the value is used to detect an update occuring. So we now
return the rowcount from the first non-trivial query that is executed
(if any). Still a slight compromise, but better than what we had.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9966 bcc190cf-cafb-0310-a4f2-bffc1f526a37