Fixed #20378 -- regression in GenericRelation on abstract model

When a GenericRelation was defined on abstract model, queries on childs
of the abstract model didn't work. The problem was in the way fields and
in particular field.rel was copied from models to their children.

The regression was likely caused by #19385. Thanks to Gavin Wahl for
spotting the regression.

tests/generic_relations_regress/models.py

@@ -122,3 +122,12 @@ class Tag(models.Model):
 
 class Board(models.Model):
     name = models.CharField(primary_key=True, max_length=15)
+
+class HasLinks(models.Model):
+    links = generic.GenericRelation(Link)
+
+    class Meta:
+        abstract = True
+
+class HasLinkThing(HasLinks):
+    pass