walkchangerevs: fix race in fast path do not yield revs if the corresponding cl entry does not exists, it avoids a race in the fastpath (where we walk in the filelog) with an ongoing transaction.

#!/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