py3: use stringutil.pprint() to prevent b'' prefixes in output

This patch uses stringutil.pprint to print a list with bytes in it to prevent
the b'' prefixes in the output.

Differential Revision: https://phab.mercurial-scm.org/D3632

mercurial/dispatch.py

@@ -288,8 +288,8 b' def _runcatch(req):'
                 req.args[2] != 'serve' or
                 req.args[3] != '--stdio'):
                 raise error.Abort(
-                    _('potentially unsafe serve --stdio invocation: %r') %
-                    (req.args,))
+                    _('potentially unsafe serve --stdio invocation: %s') %
+                    (stringutil.pprint(req.args),))
 
         try:
             debugger = 'pdb'