R1:6945e80feed5

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…