Don't require a callsign to set a repository's local path

Summary: Ref T4245. When creating new repositories, set a default local path based on the repository ID instead of callsign.

Test Plan:

• Created a new repository.
• Saw it get a reasonable, ID-based local path.
• Edited a repository to make sure the applyFinalEffects() wasn't doing anything whacky.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D15303