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

hg init a
echo a > a/a
hg --cwd a ci -A -m a
hg clone a b
echo b > b/b
hg --cwd b ci -A -m b
chmod 100 a/.hg
hg --cwd b push ../a
chmod 700 a/.hg