[MERGE] Fix pb.tick() after pb.update()

[John Arbash Meinel]

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The brisbane-core branch triggers a small bug in the new progress bars.
Basically, doing:

  pb = ui_factory.nested_progress_bar()
  pb.update('xxx', 0, 3)
  pb.tick()

Would fail. The issue is that the earlier 'update' call sets a
total_count, but the pb.tick() effectively does pb.update(...,
current=None). And the code paths weren't checking if current was None,
only if total was None.

This fixes it to exercise those code paths. It is slightly tricky,
because of the screen-update delays, but I think I've handle that fine.

John
=:->

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