R1:b66bf6af9267

When stabilizing document scroll position for diffs, stick to anchors harder

Summary: Ref T12779. Try a little harder to get the autoscroll heuristic right, but also just stick to anchors if the URL has an anchor and the scroll position is near that anchor.

Test Plan:
- Loaded an anchored diff at a bunch of window sizes, seemed pretty sticky.
- Added `usleep(100000 * mt_rand(1, 15))` to `ChangesetViewController` to make changesets load slowly and in random order, reloaded a bunch of times while…