dispatch: handle late KeyboardInterrupt occurred in run()
User can press Ctrl+C while flushing streams in dispatch.run(). In such case,
I think exiting with 255 is better than printing Python traceback and exiting
with 1.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state