R1:89fe84f97896

Add a "/source/..." URI for Diffusion commits which redirects

Summary:
See PHI112. The install presumably wants to generate links to Diffusion commits from an external tool, but only knows the short name of the repository.

Provide a `/source/phabricator/commit/abcdef908273` URI which redirects to the canonical URI for the commit.

Test Plan:
- Visited `/source/` URI for a commit, got a redirect.
- Visited normal URI for a commit, got a commit page.
- Visited `/branches/` and `/tags/` for a…