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
echo This is file a1 > a
hg add a
hg commit -m "commit #0" -d "1000000 0"
touch b
hg add b
rm b
hg commit -A -m"comment #1" -d "1000000 0"