Move generateDiffusionURI() into PhabricatorRepository

Summary: Ref T4245. This further reduces the reliance on callsigns in Diffusion.

Test Plan:

• Pretty reasonable test coverage already exists.
• Browsed repository list, browse view, history view, content view, change view, commit view, tag view, branch view of repositories.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D14937