Add prefetch_related() cache invalidation for GenericRelations

[obayemi]

with two models joined with a generic foreignkey / relationship, if we prefetch the generic relation, its cache will not be invalidated by updates (.add, .remove nor .all().delete())

class Foo(models.Model):
    tags = GenericRelation("TaggedItem")

class TaggedItem(models.Model):
    tag = models.SlugField()
    content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
    object_id = models.PositiveIntegerField()
    content_object = GenericForeignKey("content_type", "object_id")

Foo.objects.create()
f = Foo.objects.prefetch_related('tags').last()
a = TaggedItem(tag='tag')
print(f.tags.all())
f.tags.add(a, bulk=False)
print(f.tags.all())

will give

<QuerySet []>
<QuerySet []>

[Simon Charette]

This is similar to #26706 but for generic relations. The patch can probably be heavily inspired by d30febb4e59b659e0d279c77f61f936c199a05b2.

[Tom Forbes]

'Heavily inspired' PR: ​https://github.com/django/django/pull/10277

[Tim Graham <timograham@…>]

In c02d4737:

Fixed #29612 -- Added GenericRelation prefetch_related() cache invalidation.