R1:032f5b22941f

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:

{F5405517}

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