cleanups in revlog.lookup - add comments - do a clean separation of the different cases - don't use a list of each possible node when doing the lookup, just keep the previous entry

#!/bin/sh

mkdir t
cd t
hg init
echo a > a
hg add a
hg commit -m "test" -d "1000000 0"
hg verify
hg parents
hg status
hg rollback
hg verify
hg parents
hg status