Fix comitted typo in Diffusion

See: <https://github.com/facebook/phabricator/pull/468>

Reviewed by: epriestley
Fix an issue in Harbormaster when a buildable has no repository

Summary: Not every revision belongs to a repository, so we might end up here with `$repo` still equal to `null`. Don't fatal if we do.

Test Plan: iiam

Reviewers: btrahan, hach-que, zeeg

Reviewed By: hach-que

CC: aran

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D7771