Fix SVN translation of "add-file" protocol frames over SSH

Summary: Fixes T4697. When pushing moved/copied files, SVN sends an "add-file" protocol frame which has a URI in it that needs translation from external format ("/diffusion/X/") to internal format ("/path/to/svn").

Test Plan:
  - Copied/moved files and committed them in SVN.
  - Added files (no copy/move) and committed them in SVN.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4697

Differential Revision: https://secure.phabricator.com/D8654