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

echo "[extensions]" >> $HGRCPATH
echo "mq=" >> $HGRCPATH

hg init a
cd a

echo 'base' > base
hg ci -Ambase -d '1 0'

hg qnew -mmqbase mqbase

hg qsave
hg qrestore 2