For the diff banner, detect the current changeset better

Summary:
Ref T12733. Currently, we detect the changeset which is in the middle of the screen as the current changeset.

This doesn't always get us the most intuitive changeset, particularly after a navigation from the scroll objective list: when you jump to changeset "X", you'd tend to expect "X" to be shown in the header, but the next changeset may be shown if "X" is too short.

Instead, select the changeset near the top of the screen (spanning an invisible line slightly below the banner).

Test Plan: Scrolled and jumped through a document with long and short changesets, saw a more intuitive changeset selected by the banner.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12733

Differential Revision: https://secure.phabricator.com/D17976