fix the cat command - improve localrepo.walk when passed a node - make the differents walk commands in commands.py accept a node - change commands.cat to walk over a revision - add a test

#!/bin/sh

hg init a
cd a
echo a > foo
hg add foo
hg commit -m 1 -d "0 0"
rm foo
hg remove foo
hg commit -m 2 -d "0 0"

cd ..
hg clone a b