Stop stringifying None in clone kwargs

[andrewrabert]

Kwargs to git.Repo.clone and git.Repo.clone_from with None values get cast to strings and passed to the git command. This leads to behavior I'd consider non-Pythonic as it's exposing a Python internal null value to something which has no understanding of it.

For example, git.Repo.clone_from(url, path, mirror=None) results git clone --mirror=None -v url path. This is not valid as --mirror takes no arguments.

Instead, a None value should result in one of two things in the git command:

• Exclude the kwarg entirely
• Include the kwarg, but treat it as a flag without arguments (ex. --mirror without the =).