rustfmt_path invokes rustfmt with `--file-lines []`

[jsgf]

If its trying to do a full-file reformat, the file_lines vector is empty. When it constructs the command line, it ends up emitting --file-lines [], which reformats nothing.

cc @Xanewok @ljw1004

[norru]

Is it related to #1397?

[jsgf]

I think this only applies if you're using the rustfmt_path option - ie, using an external rustfmt binary rather than the library one.