[1.6.x] Fixed #20955 -- select_related regression

In cases where the same connection (from model A to model B along the
same field) was needed multiple times in a select_related query, the
join setup code mistakenly reused an existing join.

Backpatch of 8d65b6082c.

Conflicts:

	django/db/models/sql/compiler.py
	tests/queries/tests.py

tests/queries/tests.py

@@ -25,7 +25,7 @@ from .models import (
     OneToOneCategory, NullableName, ProxyCategory, SingleObject, RelatedObject,
     ModelA, ModelB, ModelC, ModelD, Responsibility, Job, JobResponsibilities,
     BaseA, FK1, Identifier, Program, Channel, Page, Paragraph, Chapter, Book,
-    MyObject, Order, OrderItem)
+    MyObject, Order, OrderItem, Task, Staff, StaffUser)
 
 
 class BaseQuerysetTest(TestCase):
@@ -2943,3 +2943,23 @@ class RelatedLookupTypeTests(TestCase):
         self.assertQuerysetEqual(
             ObjectB.objects.filter(objecta__in=[wrong_type]),
             [ob], lambda x: x)
+
+class Ticket20955Tests(TestCase):
+    def test_ticket_20955(self):
+        jack = Staff.objects.create(name='jackstaff')
+        jackstaff = StaffUser.objects.create(staff=jack)
+        jill = Staff.objects.create(name='jillstaff')
+        jillstaff = StaffUser.objects.create(staff=jill)
+        task = Task.objects.create(creator=jackstaff, owner=jillstaff, title="task")
+        task_get = Task.objects.get(pk=task.pk)
+        # Load data so that assertNumQueries doesn't complain about the get
+        # version's queries.
+        task_get.creator.staffuser.staff
+        task_get.owner.staffuser.staff
+        task_select_related = Task.objects.select_related(
+            'creator__staffuser__staff', 'owner__staffuser__staff').get(pk=task.pk)
+        with self.assertNumQueries(0):
+            self.assertEqual(task_select_related.creator.staffuser.staff,
+                             task_get.creator.staffuser.staff)
+            self.assertEqual(task_select_related.owner.staffuser.staff,
+                             task_get.owner.staffuser.staff)