message: use the newer `.get::<Option<bool>>`
diff: @@ -627,7 +627,7 @@ impl Config {
  self.cli_flags.parse(unstable_flags)?;
  if nightly_features_allowed() {
  - if let Some(val) = self.get_bool("unstable.mtime_on_use")?.map(|t| t.val) {
  + if let Some(val) = self.get::<Option<bool>>("unstable.mtime_on_use")? {
  self.cli_flags.mtime_on_use |= val;
  }
  }
  