This constructor shouldn't be public.

Also, style-nit: Line too long/wrapped incorrectly.

Also, this can only be used with a DepthWalk, which has getters for depth and compareMode.  Instead of taking a RevWalk here take a DepthWalk and get the depth and compareMode from it.  You can then downcast the DepthWalk to a RevWalk since a DepthWalk should always be a RevWalk given how your only caller inside of PendingGenerator invokes you.  :-)