Make `bin/diviner generate --repository <repository>` accept identifiers
Summary: Ref T4245. This currently accepts only callsigns; prepare it for the bright new callsign-optional world.
Test Plan:
- Ran `./bin/diviner generate --repository 1 --book src/docs/book/flavor.book --clean`, got a good result.
- Ran `./bin/diviner generate --repository 239238 --book src/docs/book/flavor.book --clean`, got an appropraite error about a bad repository identifier.
Reviewers: chad, avivey
Reviewed By: avivey
Maniphest Tasks: T4245
Differential Revision: https://secure.phabricator.com/D15296