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

@@ -27,8 +27,7 b' except ImportError:'
                      "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()