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

Maniphest Tasks: T13276

Differential Revision: https://secure.phabricator.com/D20469