Homestyx hydra

R1:a78f141b3fa8

Unify code for parsing "Reverts X" magic, and when something "reverts <hash>", also revert associated revisions

Summary:
Depends on D20468. Ref T13276. See PHI1008.

When a commit or revision "reverts <hash>", and that's the hash of a commit which has a revision, also write a "reverts" edge to the revision.

Test Plan:
Created "reverts X" objects for:

- a revision;
- a commit;
- a commit with a revision (both got marked properly).

Reviewers: amckinley

Reviewed By: amckinley

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Repository: R1 hydra
Commit Date: May 1 2019