Fix refreshing of related fields when one field is deferred

[Adam Johnson]

In #35044, we fixed the issue where loading a deferred field would clear related objects.

But it left a bug where refreshing all fields explicitly with refresh_from_db() would no longer clear related objects *if* the model instance had any deferred fields.

This would work on Django 5.0 but is broken on 5.1:

book = Book.objects.defer('title').get(pk=1)
assert book.author is not None  # fetch related object
Author.objects.all().delete()

book.refresh_from_db()
assert book.author is None  # should have been reset by the refresh

Fix incoming.

[Sarah Boyce]

I've not been able to replicate following the instructions on Django 5.1.3

[Simon Charette]

I managed to reproduce locally with ​the additional test provided in the PR.

[Sarah Boyce]

Thank you - bisected to 73df8b54a2fab53bec4c7573cda5ad8c869c2fd8
Adam can you add a release note?

[Sarah Boyce <42296566+sarahboyce@…>]

In 2f6b096b:

Fixed #35950 -- Restored refreshing of relations when fields deferred.

Thank you to Simon Charette and Sarah Boyce for the review.

Regression in 73df8b54a2fab53bec4c7573cda5ad8c869c2fd8.

[Sarah Boyce <42296566+sarahboyce@…>]

In 6e3e7353:

[5.1.x] Fixed #35950 -- Restored refreshing of relations when fields deferred.

Thank you to Simon Charette and Sarah Boyce for the review.

Regression in 73df8b54a2fab53bec4c7573cda5ad8c869c2fd8.

Backport of 2f6b096b83c55317c7ceef2d8d5dc3bee33293dc from main.