allow QuerySet.count() to query multiple collections

[timgraham]

No description provided.

[WaVEV]

I have been working on a similar task, specifically because the aggregate includes count. In my opinion, it is ok to merge this code, even though the majority of it will likely be moved or replaced.

[WaVEV]

Minor thing: Solve the TODO

[timgraham]

The todo is because there's a corresponding patch to the Django fork that I'll merge in advance of this one:

diff --git a/tests/lookup/tests.py b/tests/lookup/tests.py
index a198a13b62..aa4ed46b98 100644
--- a/tests/lookup/tests.py
+++ b/tests/lookup/tests.py
@@ -1112,6 +1112,10 @@ class LookupTests(TestCase):
         self.assertEqual(Game.objects.filter(season__gt__in=[222, 333]).count(), 5)
         self.assertEqual(Game.objects.filter(season__gt__gt=111).count(), 5)
 
+    def test_lookup_collision_distinct(self):
+        # These were separated from the above test because distinct() isn't
+        # supported.
+
         # Players who played in 2009
         self.assertEqual(
   

I don't want to merge that until this is approved since it would cause other builds to fail in the meantime.