Fixed #18854 -- Join promotion in disjunction cases

The added promotion logic is based on promoting any joins used in only
some of the childs of an OR clause unless the join existed before the
OR clause addition.

tests/regressiontests/queries/models.py

@@ -421,3 +421,21 @@ class Responsibility(models.Model):
 
     def __str__(self):
         return self.description
+
+# Models for disjunction join promotion low level testing.
+class FK1(models.Model):
+    f1 = models.TextField()
+    f2 = models.TextField()
+
+class FK2(models.Model):
+    f1 = models.TextField()
+    f2 = models.TextField()
+
+class FK3(models.Model):
+    f1 = models.TextField()
+    f2 = models.TextField()
+
+class BaseA(models.Model):
+    a = models.ForeignKey(FK1, null=True)
+    b = models.ForeignKey(FK2, null=True)
+    c = models.ForeignKey(FK3, null=True)