I haven't read all of the foregoing so forgive me if I'm repeating. Come on folky-folks we, of all people, know that nothing is new. Someone once said that there are only 3 stories and 6 tunes (or was it the other way about?) I am sure that the song was written by the reformed slave shipper (I had his name the other day in connection with something else but I'm afraid my semi-senile brain has lost it!) but the fact that he wasn't a professional composer/musician makes it even more likely that he would have borrowed a tune or the elements of a tune from elsewhere and words and phrases from things he had heard too.