config: load included config files in binary mode

I guess we've been getting lucky that this works in Python 2. This
fixes loading included config files in Python 3 (it used to fail on
the "somebody set up us the BOM" check.)

mercurial/config.py

@@ -169,5 +169,5 b' class config(object):'
 
     def read(self, path, fp=None, sections=None, remap=None):
         if not fp:
-            fp = util.posixfile(path)
+            fp = util.posixfile(path, 'rb')
         self.parse(path, fp.read(), sections, remap, self.read)