Don't require working copy to generate URIs in owners tool

Summary:
Owners packages might include repositories that are no longer tracked
(or checked out locally), and there's no reason why we need a working copy
to generate a diffusion link. Instead of displaying a red error box, this
diff allows the owners tool to render correctly.

Test Plan:
viewed /owners/view/all/ and /owners/package/395/ and verified no error box
for not having a checkout of the repository.

Reviewers: epriestley, vrana, wez

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3452