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

adding foo
adding bomb
adding a.c
adding dir/a.o
adding dir/b.o
R a.c
R dir/a.o
R dir/b.o
? .hgignore
a.c: unsupported file type (type is fifo)
R a.c