Move commit attachment to a separate CLI command

Summary:
Ref T9319. See D14967. As before, this is making a deeply-buried, complex operation easier to test by providing a CLI command.

This adds bin/differential attach-commit rXnnnn Dnnnn to pretend that rXnnnn was just committed and matched Dnnnn.

Test Plan:

• Ran bin/differential attach-commit X Y for several different values, saw updates in the UI.
• Faked the message parser to make sure stuff still worked there.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9319

Differential Revision: https://secure.phabricator.com/D14968