cat: default to working dir parent instead of tip This introduces a defaultrev function that chooses the working dir parent if a revision isn't specified, and uses it in several places.

#!/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