Allow revisions to revert commits and one another, and commits to revert revisions

Summary:
Ref T13057. This makes "reverts" syntax more visible and useful. In particular, you can now Reverts Dxx in a revision or commit, and Reverts <hash> from a revision.

When you do, the corresponding object will get a more-visible cross-reference marker in its timeline:

From here, we can look at surfacing revert information more heavily, since we can now query it on revision/commit pages via edges.

Test Plan: Used "reverts <hash>" and "reverts <revision>" in Differential and Diffusion, got sensible results in the timeline.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13057

Differential Revision: https://secure.phabricator.com/D18978