blackbox: flush output file descriptor
Without this, when there are multiple ui views, each blackbox
will have its own file handle, and the logging will be in
a really bad order.
Also, because of the way blackbox works, it never closes its
file handles, which means the last output before exit is
often lost.
Issue351: mq: qrefresh can create extra revlog entry
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ hg qinit
$ echo b > b
$ hg ci -A -m foo
adding b
$ echo cc > b
$ hg qnew -f foo.diff
$ echo b > b
$ hg qrefresh
$ hg debugindex b
rev offset length ..... linkrev nodeid p1 p2 (re)
0 0 3 ..... 0 1e88685f5dde 000000000000 000000000000 (re)