progress: stop excessive clearing (issue4801)
The progress bar was being cleared on every write(), regardless of
whether it was currently displayed. This could foul up the display of
any writes that didn't include a linebreak.
In particular, the win32 mode of the color extension was turning
single prompt string writes into two writes, and the resulting
clear/write/clear/write pattern was making the prompt invisible.
We fix this by insisting that we have shown a progress bar and haven't
just cleared it (setting lastprint to 0).
Conveniently, the test suite already had instances of duplicate
clears.. that are now cleared up.
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