ui: use pycompat.bytestr() to get a bytes-repr of config default

We don't know what type we'll get here, so we need something that'll
behave like a repr without returning a unicode.

Differential Revision: https://phab.mercurial-scm.org/D2208

mercurial/ui.py

@@ -498,7 +498,7 b' class ui(object):'
               and default != itemdefault):
             msg = ("specifying a mismatched default value for a registered "
                    "config item: '%s.%s' '%s'")
-            msg %= (section, name, default)
+            msg %= (section, name, pycompat.bytestr(default))
             self.develwarn(msg, 2, 'warn-config-default')
 
         for s, n in alternates: