R1:6a62fca95093

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…