rustdoc creates highlighted source in a directory that contains the absolute path to the source

E.g. if the crate root is located at `/home/user/lib.rs` then doing `rustdoc lib.rs` will put the highlighted source at `doc/lib/home/user/lib.rs.html`. This wasn't always the case (I estimate a few weeks ago it used relative paths). This is a problem as this can reveal more information than the user may wish to share with the rest of the world.
