[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/models.py

@@ -497,3 +497,29 @@ class OrderItem(models.Model):
 
     def __str__(self):
         return '%s' % self.pk
+
+class BaseUser(models.Model):
+    pass
+
+@python_2_unicode_compatible
+class Task(models.Model):
+    title = models.CharField(max_length=10)
+    owner = models.ForeignKey(BaseUser, related_name='owner')
+    creator = models.ForeignKey(BaseUser, related_name='creator')
+
+    def __str__(self):
+        return self.title
+
+@python_2_unicode_compatible
+class Staff(models.Model):
+    name = models.CharField(max_length=10)
+
+    def __str__(self):
+        return self.name
+
+@python_2_unicode_compatible
+class StaffUser(BaseUser):
+    staff = models.OneToOneField(Staff, related_name='user')
+
+    def __str__(self):
+        return self.staff