[Merge] Fix progress reporting in unshelve

[Martin Pool]

Thanks.

Note that this is https://bugs.edge.launchpad.net/bzr/+bug/328148 and
you should close it when you merge this and put that in the news.

> At the same time, it updates Merger.__init__ so that the default value
> is None, and DummyProgress is used in that case.  This means that other
> methods can pass in pb=None, which is more conventional, and it also
> slightly reduces the overhead of creating a DummyProgress object as the
> default argument.

I think generally we should be removing those pb= parameters that are
passed down.  Instead, let's rely on the ui object maintaining a
stack.  I'll probably send a patch for that later.

-- 
Martin <http://launchpad.net/~mbp/>