Show the oldest non-failing revision land operation, or the newest failure

Summary:
Ref T182.

• We just show the oldest operation right now, but we usually care about the oldest non-failure.
• Only query for actual land operations when rendering the revision operations dialog (maybe eventually we'll show more stuff?).
• For now, prevent multiple lands / repeated lands or queueing up lands while other lands are happening.

Test Plan: Landed a revision. Tried to land it more / again.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T182

Differential Revision: https://secure.phabricator.com/D14338