R1:74a79aa634ef

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…