[1.7.x] Fixed #22421 -- Regression in fixtures loading.

Loading fixtures were failing since the refactoring in 244e2b71f5 for
inheritance setups where the chain contains abstract models and the
root ancestor contains a M2M relation.

Thanks Stanislas Guerra for the report.

Refs #20946.

Backport of 862e1ff234 from master

tests/fixtures_regress/fixtures/feature.json

@@ -0,0 +1,17 @@
+[
+{
+  "fields": {
+    "channels": [],
+    "title": "Title of this feature article"
+  },
+  "model": "fixtures_regress.article",
+  "pk": 1
+},
+{
+  "fields": {
+    "channels": []
+  },
+  "model": "fixtures_regress.feature",
+  "pk": 1
+}
+]

tests/fixtures_regress/models.py

@@ -52,7 +52,7 @@ class Child(Parent):
     data = models.CharField(max_length=10)
 
 
-# Models to regression test #7572
+# Models to regression test #7572, #20820
 class Channel(models.Model):
     name = models.CharField(max_length=255)
 
@@ -70,6 +70,17 @@ class SpecialArticle(Article):
     pass
 
 
+# Models to regression test #22421
+class CommonFeature(Article):
+
+    class Meta:
+        abstract = True
+
+
+class Feature(CommonFeature):
+    pass
+
+
 # Models to regression test #11428
 @python_2_unicode_compatible
 class Widget(models.Model):

tests/fixtures_regress/tests.py

@@ -480,6 +480,18 @@ class TestFixtures(TestCase):
             verbosity=0,
         )
 
+    def test_ticket_22421(self):
+        """
+        Regression for ticket #22421 -- loaddata on a model that inherits from
+        a grand-parent model with a M2M but via an abstract parent shouldn't
+        blow up.
+        """
+        management.call_command(
+            'loaddata',
+            'feature.json',
+            verbosity=0,
+        )
+
 
 class NaturalKeyFixtureTests(TestCase):