Allow revisions to revert commits and one another, and commits to revert revisions master
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: {F5405517} 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