Make serving repositories work with alternate URIs

Summary: Ref T4245. Consolidates the URI parsing/rewriting logic so that repositories can be served from either /diffusion/XYZ/ or /diffusion/123/, over both HTTP and SSH.

Test Plan:

• Pulled a Git repository by ID and callsign over HTTP and SSH.
• Pulled a Mercurial repository by ID and callsign over HTTP and SSH.
• Pulled a Subversion repository by ID and callsign over SSH (no HTTP support for SVN).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D15302