x.py fmt silently ignores argument

[RalfJung]

Ever since rustfmt was introduced into this repository, I used ./x.py fmt src/librustc_mir and similar commands to format only that part of the tree, thinking that this would cut down formatting time a bit. But I just realized that even when I do ./x.py fmt src/asdfgh, it just formats the entire repository. Looks like the argument is being silently ignored.

Ideally the argument should be honored, but at the least, there should be an error when passing an unexpected argument.