A new graph instance should be passed to `iterate`

[JLHwung]

On the iterate step, the graph g is passed to iterate. However, as ContractEdge mutates the graph, it will render contracted g passed to iterate on the second and all the following rounds.

https://github.com/chasestarr/mincut/blob/master/mincut.go#L17 should have been something like

if m := iterate(g.Clone(), listedTwice); m < min {
    min = m
}