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

src/repository/api/ArcanistSubversionAPI.php

@@ -651,7 +651,7 @@ EODIFF;
     $results = $conduit->callMethodSynchronous('differential.query', $query);
 
     foreach ($results as $key => $result) {
-      if ($result['sourcePath'] != $this->getPath()) {
+      if (idx($result, 'sourcePath') != $this->getPath()) {
         unset($results[$key]);
       }
     }