Fix an issue with SVN path construction in the presence of subpath configuration

Summary: D7590 made path construction more consistent, but affected this callsite if a subpath is configured. Currently, we end up with double `@@` in the URI.

Test Plan:
  - Ran unit tests.
  - Ran `bin/repostitory discover`.

Reviewers: staticshock, btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7619