Arcanist - add revision data to TYPE_LAND_WILLPUSHREVISION event

Summary:
pretty straight-forward stuff here. Note that in other events we use "fields" or "specification" rather than "revision"; I think "revision" is best particularly in this context where it is in fact the revision being landed.

Fixes T4565.

Test Plan: php -l

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: aran, epriestley, Korvin

Maniphest Tasks: T4565

Differential Revision: https://secure.phabricator.com/D8484