Instead of looping forever, resolve the relative commit

Summary: Obvious error in refactoring code around getCanonicalRevisionName() in D1954.

Test Plan: Ran "arc diff" without looping. Can you verify this fixes your case?

Reviewers: Makinde, btrahan

Reviewed By: Makinde

CC: aran, epriestley

Maniphest Tasks: T1025

Differential Revision: https://secure.phabricator.com/D1970

src/repository/api/mercurial/ArcanistMercurialAPI.php

@@ -61,8 +61,7 @@ final class ArcanistMercurialAPI extends ArcanistRepositoryAPI {
     list($stdout) = $this->execxLocal(
       'log -l 1 --template %s -r %s --',
       '{node}',
-      $this->getRelativeCommit());
-
+      $string);
     return $stdout;
   }