dirstate: add __contains__ and make __getitem__ more useful
dirstate.state(f) == '?' -> f not in dirstate
dirstate.state(f) -> dirstate[f]
adding dir/bar
adding foo
dir/bar
foo
adding dir/bar_2
adding foo_2
dir/bar_2
foo_2
adding a
adding c
adding b
adding d
removing a
removing c
recording removal of a as rename to b (100% similar)