Write edges for commit reverts

Summary:
Ref T1751. When a commit reverts another commit:

• Add an edge linking them;
• Show the edge in Diffusion.

Next steps are:

• If the reverted commit is associated with a Differential revision, leave a comment;
• Also leave a comment on the commit (no API yet);
• Also trigger an audit by the original commit's author.

Test Plan: Used scripts/repository/reparse.php --message ... to parse commits with revert language. Verified they appear correctly in Diffusion, and update Differential.

Reviewers: btrahan, epriestley

Reviewed By: btrahan, epriestley

Subscribers: Korvin, epriestley, cburroughs, joshuaspence, sascha-egerer, aran

Maniphest Tasks: T4896, T1751

Differential Revision: https://secure.phabricator.com/D5846