Commit#getDiff callback is undocumented

[JC3]

The documentation at https://www.nodegit.org/api/commit/#getDiff does not document the callback for getDiff (the parameters it takes, etc.).

Additionally, the API doesn't seem to make sense: It takes a callback and returns a promise. It's unclear what the callback is for in the first place, as it seems to be an odd mix of promise-based and callback-based asynchronicity, which is typically either one or the other.