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

unset HGUSER
EMAIL="My Name <myname@example.com>"
export EMAIL

hg init test
cd test
touch asdf
hg add asdf
hg commit -d '1000000 0' -m commit-1
hg tip