Fix all remaining weird Diffusion request processing

Summary: Ref T4245. This is the last of it, and covers the clone/push stuff.

Test Plan:

• Cloned git.
• Pushed git.
• Cloned mercurial.
• Pushed mercurial.
• Visited a blah.git URL in my browser just because; got redirected to a human-facing UI.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D14949