Can you commonize the two `GetCommitMessage` methods? In particular, the last line, but I would guess that something like this would work for the 1-argument version:

```
public string GetCommitMessage(string commitish)
{
    return GetCommitMessage(commitish, commitish + "^");
    // or, GetCommitMessage(commitish, _repository.Lookup<Commit>(commitish).Parents.First().Id);
}
```