cmdutil: changeset_printer: use methods of filectx/changectx. This allows extensions that modify changeset metadata (e.g. description) by overriding methods of changectx to get consistent behavior from all log-like commands, regardless of whether templates or styles are used. Without this, overriding changectx methods works if you use styles or templates, but not with default log format. This meant adding filectx.extra() for consistency with changectx.