Increase clarity when closing a revision in response to a commit

Summary:
I am not sure how valuable this is *as is* - I think it needs different explanations for what happened in mercurial or subversion? I do not know what those explanations are.

Made an error in D10485 - the $hashes that were saved is an array of objects, so it ends up turning into garbage via the wonders of serialization and de-serialization. Fix that by explicitly saving the tree hash.

I would like to make this work for the other VCS types we support, add the "undo / nope" button and call it fixed.

Ref T3686.

Test Plan: clicked "explan why" and saw why

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5693, T3686

Differential Revision: https://secure.phabricator.com/D10489