Arcanist - potentially fix some Windows badness

Summary: Fixes T5914, maybs.

Test Plan: will ask users in T5914

Reviewers: chad, epriestley

Reviewed By: chad, epriestley

Subscribers: chad, bluehawk, epriestley, Korvin

Maniphest Tasks: T5914

Differential Revision: https://secure.phabricator.com/D10333

src/repository/api/ArcanistGitAPI.php

@@ -535,7 +535,9 @@ final class ArcanistGitAPI extends ArcanistRepositoryAPI {
       $stdout = $this->getHashFromFromSVNRevisionNumber($match[1]);
     } else {
       list($stdout) = $this->execxLocal(
-        'show -s --format=%s %s --',
+        phutil_is_windows()
+        ? 'show -s --format=%C %s --'
+        : 'show -s --format=%s %s --',
         '%H',
         $string);
     }