Use unsigned version format. This way can use one additional bit, and when encountering invalid revlogs with the first bit set don't produce python warnings or strange error messages.

#!/bin/sh

hg init
mkfifo p

hg serve --stdio < p &
P=$!
(echo lock; echo addchangegroup; sleep 5) > p &
Q=$!
sleep 3
kill -HUP $P
wait
ls .hg