Given a value spelled "algo:digest", append the digest to a list
    pointed to in a dict by the algo name.

def _merge_hash(option, opt_str, value, parser):
    # type: (Option, str, str, OptionParser) -> None
    """Given a value spelled "algo:digest", append the digest to a list
    pointed to in a dict by the algo name."""
    if not parser.values.hashes:
        parser.values.hashes = {}  # type: ignore
    try:
        algo, digest = value.split(':', 1)
    except ValueError:
        parser.error('Arguments to %s must be a hash name '
                     'followed by a value, like --hash=sha256:abcde...' %
                     opt_str)
    if algo not in STRONG_HASHES:
        parser.error('Allowed hash algorithms for %s are %s.' %
                     (opt_str, ', '.join(STRONG_HASHES)))
    parser.values.hashes.setdefault(algo, []).append(digest)