This is a thread that died away with question I asked in it unanswered - specifically, how old is this song, and does it go back to slave times, or was it a more recent creation.
Anyway, searching for something else, I came across this post that goes some way to answer that. The song was evidently in a collection of slave songs published in 1867.
But it'd still be interesting to know how far it has been in the oral tradition over the next few generations, and whether there are variants which have been collected.