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