Fixed #31071 -- Disabled insert optimization for primary keys with defaults when loading fixtures.

Model.save_base() is called directly when loading fixtures and assumes
existing rows will be updated. Branching of "raw" allows to maintain
the optimization introduced in #29260 while supporting this edge case.

Regression in 85458e94e3.

Thanks Reupen Shah for the report.

django/db/models/base.py

@@ -849,6 +849,7 @@ class Model(metaclass=ModelBase):
         updated = False
         # Skip an UPDATE when adding an instance and primary key has a default.
         if (
+            not raw and
             not force_insert and
             self._state.adding and
             self._meta.pk.default and