You shouldn't do this.  You can't start a RevCommit into a different pool than it was parsed out of.

You should be using the internal RevWalk of this class.  Just reset it, enqueue the start, and then run.

But this makes no sense.  Near as I can tell the walk is only used to parse other commits.  Its not actually used for a traversal.  So marking a starting point, or setting its sorting to TOPO is pointless.