R1:85a363674743

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…