Don't use absolute path in ArcanistDiffUtils

Summary: Improves Windows compatibility.

Test Plan: Ran failing unit test on Windows.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5344

src/difference/ArcanistDiffUtils.php

@@ -38,7 +38,7 @@ public static function renderDifferences(
     Filesystem::writeFile($file_new, (string)$new."\n");
 
     list($err, $stdout) = exec_manual(
-      '/usr/bin/diff %C -U %s %s %s',
+      'diff %C -U %s %s %s',
       $diff_options,
       $context_lines,
       $file_old,