If an SQL query doesn't specify any ordering, avoid the implicit sort

that happens with MySQL when a "GROUP BY" clause is included. This is a
backend-specific operation, so any other databases requiring similar
encouragement can have a function added to their own backend code.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@9637 bcc190cf-cafb-0310-a4f2-bffc1f526a37