Represent one jupytext format as a string

def short_form_one_format(jupytext_format):
    """Represent one jupytext format as a string"""
    if not isinstance(jupytext_format, dict):
        return jupytext_format
    fmt = jupytext_format['extension']
    if 'suffix' in jupytext_format:
        fmt = jupytext_format['suffix'] + fmt
    elif fmt.startswith('.'):
        fmt = fmt[1:]

    if 'prefix' in jupytext_format:
        fmt = jupytext_format['prefix'] + '/' + fmt

    if jupytext_format.get('format_name'):
        if jupytext_format['extension'] not in ['.md', '.Rmd'] or jupytext_format['format_name'] == 'pandoc':
            fmt = fmt + ':' + jupytext_format['format_name']

    return fmt