Revert changeset c67920d78248. It breaks 'hg merge && hg up -c REV' when files exist only in one of the manifests, making hg throw a traceback like:

#!/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