Add "diffusion.commit.edit", a v3 edit API endpoint for commits

Summary: Ref T10978. This currently does almost nothing, but gets it in place so I can add stuff to it.

Test Plan: Made a comment on a commit using the API.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10978

Differential Revision: https://secure.phabricator.com/D17178