Support slightly prettier repository URIs in Diffusion

Summary: Fixes T4245. When a repository has a short name, use /source/shortname/ as its primary URI.

Test Plan:

• Cloned Git repositories from shortnames via HTTP and SSH.
• Cloned Mercurial repositories from shortnames via HTTP and SSH.
• Cloned Subversion repositories from shortnames via SSH.
• Browsed Git, Mercurial and Subversion repositories.
• Added and removed short names to various repositories.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D16851