Stabilize scroll position as diffs load

Summary:
Try to lock the screen to whatever the user is looking at as we load changesets.

Notably, this improves the use case of taking a known action on a diff. Currently, you have to wait for everything to load or the comments keep getting scrolled down. After this change, the comments stay in the same place on screen.

Test Plan:
Raised the autoload changeset limit from 100 to 1000, looked at a 220 changeset diff.

  - Reloaded it while scrolled at the top; normal behavior (no scrolling).
  - Reloaded it, scrolled to the bottom. Comment area now stable.
  - Reloaded it, kind of scrolled around the middle? Behavior seemed stable/reasonable. This one is kind of heursitic so it's hard to say I'm getting it totally right or not, but it's less important than the "bottom" case.

Reviewers: vrana, btrahan, chad, dctrwatson

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D4774