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
1:3aa14bbc23d90:8633637036c13aa14bbc23d9 tip8633637036c18633637036c1+8633637036c13aa14bbc23d9 tipmerging file13aa14bbc23d9 tip3aa14bbc23d9 tip3aa14bbc23d9 tip3aa14bbc23d9 tip