Fixed #25939 -- Removed redundant transaction in QuerySet.update_or_create().

There is no need to wrap the save() call in transaction.atomic() as
it's already done down the call stack in Model.save_base().

django/db/models/query.py

@@ -484,9 +484,7 @@ class QuerySet(object):
                 return obj, created
         for k, v in six.iteritems(defaults):
             setattr(obj, k, v)
-
-        with transaction.atomic(using=self.db, savepoint=False):
-            obj.save(using=self.db)
+        obj.save(using=self.db)
         return obj, False
 
     def _create_object_from_params(self, lookup, params):