make ui flush output. this makes error happen if printing to /dev/full. before this change, error happened with some pythons, but not others. now always happens. fixes issue 155.

#!/bin/sh

mkdir t
cd t
hg init
echo a > a
hg add a
hg commit -m "test" -d "0 0"
hg verify
hg parents
hg status
hg undo
hg verify
hg parents
hg status