hg: add support for HGUNICODEPEDANTRY environment variable This lets us easily verify that there are no implicit conversions between unicodes and bytes in Mercurial's codebase. Based on something mpm did by hand periodically, but it kept regressing, so just open the door to running it in a buildbot.

--- a/hg	Tue May 27 21:02:16 2014 -0700
+++ b/hg	Mon Jun 23 09:33:07 2014 -0400
@@ -10,6 +10,11 @@
 import os
 import sys
 
+if os.environ.get('HGUNICODEPEDANTRY', False):
+    reload(sys)
+    sys.setdefaultencoding("undefined")
+
+
 libdir = '@LIBDIR@'
 
 if libdir != '@' 'LIBDIR' '@':