R1:413ca12fda6b

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…