message: Test ensure_directory_name_unique with long name
  My first suspicion for bug 272 was that the ensure_directory_name_unique
  function was misbehaving when names hit the directory size limit.
  I wrote a test to check that hypothesis. The test proved me wrong. I
  thought it may be worth committing it to document this correct
  behaviour.
diff: @@ -777,4 +777,15 @@ mod test {
  assert_eq!("group/function/value_3", new_id.as_directory_name());
  directories.insert(new_id.as_directory_name().to_owned());
  }
  + #[test]
  + fn test_benchmark_id_make_long_directory_name_unique() {
  + let long_name = (0..MAX_DIRECTORY_NAME_LEN).map(|_| 'a').collect::<String>();
  + let existing_id = BenchmarkId::new(long_name, None, None, None);
  + let mut directories = HashSet::new();
  + directories.insert(existing_id.as_directory_name().to_owned());
  +
  + let mut new_id = existing_id.clone();
  + new_id.ensure_directory_name_unique(&directories);
  + assert_ne!(existing_id.as_directory_name(), new_id.as_directory_name());
  + }
  }
  