Clean up NUX a bit on Diffusion

Summary: Just some cleanup. Make sure action-bar has consistent space if locate is there or not, hide tabs if repository has no content. Use clone or checkout language depending on SCM. Fixes T12915.

Test Plan:
Test git, hg, svn blank states.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12915

Differential Revision: https://secure.phabricator.com/D18208