Improve Differential behavior when scrolling with anchors

Summary:
Fixes T12779. Currently, we scroll down if the midline of the changeset is above the midline of the viewport.

This rule can cause us to scroll improperly when loading changesets after jumping to their anchors, since the changeset we want to look at will likely have a midpoint above the document midline. That is, we follow an anchor to X.c, then it loads, then we scroll past it.

Instead, scroll only if the changeset is (almost) entirely above the viewport.

Test Plan:
Followed an anchor to PHUIFeedStoryExample:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12779

Differential Revision: https://secure.phabricator.com/D18052