Backwards incompatible if you are using manually-specific primary key values
and relying on the previously documented behaviour that the new values would
always exist in the database (i.e. it would update the existing entry).
Fixed#8419.
git-svn-id: http://code.djangoproject.com/svn/django/trunk@8670 bcc190cf-cafb-0310-a4f2-bffc1f526a37