[Merge] Fix progress reporting in unshelve

[Aaron Bentley]

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Hi all,

This fixes the unshelve warnings we've been getting:

/home/abentley/bzr/bzr.dev/bzrlib/ui/text.py:99: UserWarning:
ProgressTask(0/3, msg='Merge phase') is not the top progress task
ProgressTask(None/None, msg='')
  (task, self._task_stack[-1]))

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.

Aaron
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