Fix an issue with viewing an undiscovered commit in Diffusion

Summary: If you load Diffusion between a repository being pulled and discovered, you can end up with a valid commit reference that hasn't been discovered yet. Don't fatal.

Test Plan: Saw somewhat-helpful error page instead of fatal.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7448