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

hg init
echo This is file a1 > a
hg add a
hg commit -m "commit #0" -d "0 0"
touch b
hg add b
rm b
hg commit -A -m"comment #1" -d "0 0"