Fix an issue where scrolling down, then up, then down fails to show changeset header in Differential

Summary: Ref T13151. See PHI616. There's a bug where the current banner changeset isn't cleared correctly when we hide the banner.

Test Plan:

• View revision with several changesets.
• Scroll down slowly through first changeset until banner appears.
• Scroll up until banner disappears.
• Scroll back down.
• Before: banner fails to reappear (code still thinks it's visible and we don't want to update it).
• After: banner reappears correctly.

Reviewers: amckinley, jmeador

Reviewed By: jmeador

Maniphest Tasks: T13151

Differential Revision: https://secure.phabricator.com/D19474