Formats a percent field.

def format_percent_field(__, prec, number, locale):
    """Formats a percent field."""
    prec = PERCENT_DECIMAL_DIGITS if prec is None else int(prec)
    locale = Locale.parse(locale)
    pattern = locale.percent_formats.get(None)
    return pattern.apply(number, locale, force_frac=(prec, prec))