Undiscard DATASETS previously discarded.

def marvcli_undiscard(datasets):
    """Undiscard DATASETS previously discarded."""
    create_app()

    setids = parse_setids(datasets, discarded=True)
    dataset = Dataset.__table__
    stmt = dataset.update()\
                  .where(dataset.c.setid.in_(setids))\
                  .values(discarded=False)
    db.session.execute(stmt)
    db.session.commit()