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 /source/ repository, got proper pages.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18676