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

hg init rep
cd rep
mkdir dir
touch foo dir/bar
hg -v addremove
hg -v commit -m "add 1" -d "1000000 0"
cd dir/
touch ../foo_2 bar_2
hg -v addremove
hg -v commit -m "add 2" -d "1000000 0"