[1.8.x] Fixed #24748 -- Fixed incorrect GROUP BY on MySQL in some queries

When the query's model had a self-referential foreign key, the
compiler.get_group_by() code incorrectly used the self-referential
foreign key's column (for example parent_id) as GROUP BY clause
when it should have used the model's primary key column (id).

Backport of adc57632bc from master

tests/aggregation_regress/models.py

@@ -104,3 +104,8 @@ class Bravo(models.Model):
 class Charlie(models.Model):
     alfa = models.ForeignKey(Alfa, null=True)
     bravo = models.ForeignKey(Bravo, null=True)
+
+
+class SelfRefFK(models.Model):
+    name = models.CharField(max_length=50)
+    parent = models.ForeignKey('self', null=True, blank=True, related_name='children')