add Haskell extensions to ##differential.whitespace-matters## in default.conf.py

Summary: ##/\.l?hs$/## files are sensitive to changes in whitespace

Test Plan: what, me worry?

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1482

Author: cpiro

conf/default.conf.php

@@ -592,6 +592,7 @@
   // use 'ignore-all' by default
   'differential.whitespace-matters' => array(
     '/\.py$/',
+    '/\.l?hs$/',
   ),
 
   'differential.field-selector' => 'DifferentialDefaultFieldSelector',