progress: flush stderr after clearing

On python3, ui.stderr is buffered, it seems, so we need to flush it to actually
get the progress bar off the screen. This is important since ui.write() will
call into progbar.clear() if it thinks there's a progress bar on the screen,
with the intent that the next thing it outputs is at the beginning of the line
(instead of at the end of the progress bar line). Without the flush, we buffer
up the clearing of the screen, and we get some really weird/corrupt output.

Differential Revision: https://phab.mercurial-scm.org/D7784

mercurial/progress.py

@@ -193,6 +193,7 b' class progbar(object):'
         if not self.printed or not self.lastprint or not shouldprint(self.ui):
             return
         self._writeerr(b'\r%s\r' % (b' ' * self.width()))
+        self._flusherr()
         if self.printed:
             # force immediate re-paint of progress bar
             self.lastprint = 0