emacs: simplify datastring type in update-from-overlay.py

def commit_fileset(

    name: str,

    sha: str,

    datestring: str | None,

    datestring: str,

    emacs_overlay: EmacsOverlay,

    here_directory: HereDirectory,

    logger: logging.Logger,

    Arguments:

    name -- The name of fileset.

    sha -- The commit SHA of emacs-overlay from which the files are retrieved.

    datestring -- The date to be written on commit message. If None, use the current time.

    datestring -- The date to be written on commit message.

    """

    logger.debug("BEGIN")

    nix_attrs = emacs_overlay.elisp_packages_set[name]["nix_attrs"]

    basename = emacs_overlay.elisp_packages_set[name]["basename"]

    if datestring is None:

        datestring = datetime.datetime.today().strftime("%Y-%m-%d")

        logger.debug(f"Date string not provided, using {datestring}")

    else:

        logger.debug(f"Date string was provided: {datestring}")

    logging.debug(f"Date: {datestring}")

    cmdline_verify = ["git", "diff", "--exit-code", "--quiet", "--", basename]

    result_verify = subprocess.run(cmdline_verify)