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
hg -q
hg help
hg -q help
hg add -h
hg add --skjdfks
hg help diff
hg help status
hg -q help status
hg help foo
hg skjdfks

exit 0