Added savepoint protection to get_or_create() to avoid problems on PostgreSQL.

Fixed #7402.

Also made savepoint handling easier to use when wrapped around calls that might
commit a transaction. This is tested by the get_or_create tests.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8315 bcc190cf-cafb-0310-a4f2-bffc1f526a37