cmdutil: use None as default value for "function pointer" instead of False

Less type confusion.

mercurial/cmdutil.py

@@ -933,7 +933,7 b' def show_changeset(ui, repo, opts, buffe'
     regular display via changeset_printer() is done.
     """
     # options
-    patch = False
+    patch = None
     if opts.get('patch') or opts.get('stat'):
         patch = scmutil.matchall(repo)