Fixed #13156 -- Ensure that exists() queries are as fast as they can be....

Fixed #13156 -- Ensure that exists() queries are as fast as they can be. Thanks to Jerome Leclanche for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12810 bcc190cf-cafb-0310-a4f2-bffc1f526a37