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.

hg

@@ -10,6 +10,11 b''
 import os
 import sys
 
+if os.environ.get('HGUNICODEPEDANTRY', False):
+    reload(sys)
+    sys.setdefaultencoding("undefined")
+
+
 libdir = '@LIBDIR@'
 
 if libdir != '@' 'LIBDIR' '@':