Code review comment for lp:~mbp/bzr/cleanup

I did find this useful later, so on the weight of spiv's earlier +Approved I'm going to send it up to pqm.

I agree that often one would want "do this and then clean up", but I think you can also have cases where cleanup_now will be called from eg a tearDown method, and it's somebody else's job to arrange for that to be called.