.select_related() doesn't fetch `ReferenceField`

If you have multiple ReferenceFields to the same model – .select_related() doesn't fetch all the references it's supposed to.

This was also brought up in PR #1226, I believe.

Here's a test that shows the behavior (from the same PR):

    def test_select_related_follows_referencefields(self):
        class Human(Document):
            pass

        class Car(Document):
            owner = ReferenceField(Human)

        class ParkingSlot(Document):
            owner = ReferenceField(Human)
            car = ReferenceField(Car)

        Human.drop_collection()
        Car.drop_collection()
        ParkingSlot.drop_collection()

        for _ in range(24):
            owner = Human.objects.create()
            car = Car.objects.create(owner=owner)
            ParkingSlot.objects.create(owner=owner, car=car)

        with query_counter() as q:
            self.assertEqual(q, 0)
            parking_slots = ParkingSlot.objects.select_related(max_depth=2)
            self.assertEqual(q, 3)
            for parking_slot in parking_slots:
                parking_slot.owner
                parking_slot.car.owner
            self.assertEqual(q, 3) # <-- q is 27 here!

Start with the provided test_select_related_follows_referencefields scenario and the select_related(max_depth=2) entry point. Reproduce the query-count failure with multiple ReferenceFields, then make the test pass so accessing parking_slot.owner and parking_slot.car.owner performs no additional queries beyond the initial three.