message: use postfix negation instead of prefix
diff: @@ -4,6 +4,7 @@ use std::fs::{self, File};
  use std::io::{self, Write, BufRead};
  use std::path::{PathBuf, Path};
  use std::process::Command;
  +use std::ops::Not;
  const CARGO_MIRI_HELP: &str = r#"Interprets bin crates and tests in Miri
  @@ -250,9 +251,9 @@ fn setup(ask_user: bool) {
  }
  // FIXME: Install from crates.io again once a new xargo got released.
  - if !cargo().args(&["install", "xargo", "-f", "--git", "https://github.com/japaric/xargo"]).status()
  + if cargo().args(&["install", "xargo", "-f", "--git", "https://github.com/japaric/xargo"]).status()
  .expect("failed to install xargo")
  - .success()
  + .success().not()
  {
  show_error(format!("Failed to install xargo"));
  }
  @@ -326,9 +327,9 @@ path = "lib.rs"
  command.arg("--target").arg(&target);
  }
  // Finally run it!
  - if !command.status()
  + if command.status()
  .expect("failed to run xargo")
  - .success()
  + .success().not()
  {
  show_error(format!("Failed to run xargo"));
  }
  