Fix revlog.parseindex parseindex could fail if read returns too little data in the right moment (e.g. when there's still leftover data from the previous iteration and read returns less than "s" bytes).

#!/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