Fix arc which for svn repos

Summary:
arc which is currently broken for svn repos.

Fixes T6635

(I think I independently wrote an identical change to yours)

Test Plan: ∴../../p/arcanist/bin/arc which on a svn repo.

Reviewers: aik099, epriestley, #blessed_reviewers

Reviewed By: aik099, epriestley, #blessed_reviewers

Subscribers: aik099, Korvin

Maniphest Tasks: T6635

Differential Revision: https://secure.phabricator.com/D15922