message: Checkout specific tags of NSS and NSPR
  Checking out the tip of these, NSS especially, can cause mysterious
  breakage that's not our fault. Instead of the tip, check out recent RTM
  tags.
diff: @@ -99,6 +99,13 @@ fn nss_dir() -> PathBuf {
  ])
  .status()
  .expect("can't clone nss");
  + let orig_dir = env::current_dir().unwrap();
  + env::set_current_dir(&dir).unwrap();
  + Command::new("hg")
  + .args(&["up", "NSS_3_50_RTM"])
  + .status()
  + .expect("can't update to tag for NSS");
  + env::set_current_dir(&orig_dir).unwrap();
  }
  let nspr_dir = Path::new(&out_dir).join("nspr");
  if !nspr_dir.exists() {
  @@ -110,6 +117,13 @@ fn nss_dir() -> PathBuf {
  ])
  .status()
  .expect("can't clone nspr");
  + let orig_dir = env::current_dir().unwrap();
  + env::set_current_dir(&nspr_dir).unwrap();
  + Command::new("hg")
  + .args(&["up", "NSPR_4_25_RTM"])
  + .status()
  + .expect("can't update to tag for NSPR");
  + env::set_current_dir(&orig_dir).unwrap();
  }
  dir.to_path_buf()
  };
  