Why does this variant of the constructor disable grafts, while another enables them?  This will result in a lot of confusion in the current code because some code uses this constructor just to reuse an existing reader between different related objects.

I know the problem is you don't have the graft data available in this context.  But its still scary that the behavior of RevWalk differs based on the type of object we used to create it, and the Javadoc doesn't reflect this.