R1:e7a7e43104bc

Fix a bug where policy queries with cursor-based pagers and non-ID orders can go into infinite loops

Summary:
Ref T603. See inlines for an explanation. The case where I hit this was loading the "Pending Differential Revisions" panel in Diffusion when logged out, after making a repository public.

What happens is that we load 10 revisions (say, D1 .. D10) but the user can't see any of them. We then try to load the next 10, but since the pagination is ordered by date modified, we need to base the next query…