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.
This patch changes "progress.shouldprint()" so a feature name is provided to
"ui.plain()" to determine if there is an exception specificed in HGPLAINEXCEPT
for the progress extension.
This will allow user-facing scripts to provide progress output while HGPLAIN
is enabled.
This avoids some visual flickering of the progress bar in convert and
probably some other operations. Previously, a line of output would
erase the progress bar, and then it would wait `progress.refresh`
seconds (default of 0.1) before redrawing the progress bar. Now if
we've ever painted a progress bar, we schedule the progress bar for
immediate repainting on the next progress call, which helps lend the
illusion that the progress bar is "always" there. In practice, it's
merely there more of the time, but it ends up being a lot easier to
read during convert.
