history.py uses deprecated (and deleted) ProgressBarStack

history.py:

class ThreadSafeUIFactory (bzrlib.ui.SilentUIFactory):

    def nested_progress_bar(self):
        if getattr(uihack, '_progress_bar_stack', None) is None:
            pbs = bzrlib.progress.ProgressBarStack(
                      klass=bzrlib.progress.DummyProgress)
            uihack._progress_bar_stack = pbs
        return uihack._progress_bar_stack.get_nested()

This causes a DeprecationWarning:

.../loggerhead/history.py:63: DeprecationWarning: bzrlib.progress.ProgressBarStack was deprecated in version 1.12.
  klass=bzrlib.progress.DummyProgress)

lp:~beuno/loggerhead/deprecate should be updated to fix this.