external_subcommand triggers unused_braces lint

[imp]

Taken verbatim from the example:

#[derive(Debug, StructOpt)]
enum Command {
    #[structopt(external_subcommand)]
    Other(Vec<String>),
}

upsets compiler (probably new lint)

  --> src/main.rs:81:15
   |
81 |     Other(Vec<String>),
   |               ^^^^^^ help: remove these braces
   |
note: the lint level is defined here
  --> src/main.rs:14:9
   |
14 | #![warn(unused)]
   |         ^^^^^^
   = note: `#[warn(unused_braces)]` implied by `#[warn(unused)]`

[CreepySkeleton]

Fixed by #399