R1:9febfb26a08d

Convert `diffusion.looksoon` to use repository identifiers instead of callsigns

Summary:
Ref T4245. Like everything else, accept more identifiers.

This needs a change in `arc`, which I've made a note about elsewhere.

Test Plan: Used "Update Now" from web UI, saw update get scheduled.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D14932