message: exec (replace the current process) external subommands instead of running them as child processes.
diff: @@ -325,7 +325,7 @@ fn execute_external_subcommand(config: &Config, cmd: &str, args: &[String]) -> C
  let err = match util::process(&command)
  .env(cargo::CARGO_ENV, cargo_exe)
  .args(&args[1..])
  - .exec() {
  + .exec_replace() {
  Ok(()) => return Ok(()),
  Err(e) => e,
  };
  