doc: don't use mutable default arguments

It appears our mutable default arguments checker doesn't find
arguments not on the first line of a function definition :(

The arguments aren't used, so the default value is irrelevant.

I found this when blackening this script in a future commit.

Differential Revision: https://phab.mercurial-scm.org/D7445

doc/runrst

@@ -1,64 +1,63 b''
 #!/usr/bin/env python
 #
 # runrst - register custom roles and run correct writer
 #
 # Copyright 2010 Matt Mackall <[e-mail]> and others
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 """usage: %s WRITER args...
 
 where WRITER is the name of a Docutils writer such as 'html' or 'manpage'
 """
 
 from __future__ import absolute_import
 
 import sys
 try:
     import docutils.core as core
     import docutils.nodes as nodes
     import docutils.utils as utils
     import docutils.parsers.rst.roles as roles
 except ImportError:
     sys.stderr.write("abort: couldn't generate documentation: docutils "
                      "module is missing\n")
     sys.stderr.write("please install python-docutils or see "
                      "http://docutils.sourceforge.net/\n")
     sys.exit(-1)
 
-def role_hg(name, rawtext, text, lineno, inliner,
-            options={}, content=[]):
+def role_hg(name, rawtext, text, lineno, inliner, options=None, content=None):
     text = "hg " + utils.unescape(text)
     linktext = nodes.literal(rawtext, text)
     parts = text.split()
     cmd, args = parts[1], parts[2:]
     refuri = "hg.1.html#%s" % cmd
     if cmd == 'help' and args:
         if args[0] == 'config':
             # :hg:`help config`
             refuri = "hgrc.5.html"
         elif args[0].startswith('config.'):
             # :hg:`help config.SECTION...`
             refuri = "hgrc.5.html#%s" % args[0].split('.', 2)[1]
         elif len(args) >= 2 and args[0] == '-c':
             # :hg:`help -c COMMAND ...` is equivalent to :hg:`COMMAND`
             # (mainly for :hg:`help -c config`)
             refuri = "hg.1.html#%s" % args[1]
         else:
             refuri = "hg.1.html#%s" % args[0]
     node = nodes.reference(rawtext, '', linktext,
                            refuri=refuri)
     return [node], []
 
 roles.register_local_role("hg", role_hg)
 
 if __name__ == "__main__":
     if len(sys.argv) < 2:
         sys.stderr.write(__doc__ % sys.argv[0])
         sys.exit(1)
 
     writer = sys.argv[1]
     del sys.argv[1]
 
     core.publish_cmdline(writer_name=writer)