Use the Git class type definition within Repo classmethods

[mmulich]

Allow the GitCommandWrapperType definition to be used within the Repo
classmethods. This change follows the intended purpose as stated in
the code, "Subclasses may easily bring in their own custom types by
placing a constructor or type here."

The usecase that prompted this change has to do with
GIT_SSH_COMMAND. The goal is to setup a custom Git class with
knowledge of the value, something like as follows

from git import Git as BaseGit, Repo as BaseRepo

class Git(BaseGit):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        # For example, assign the SSH command using the current flask
        # app's configured setting.
        self.update_environment(GIT_SSH_COMMAND=current_app.config['GIT_SSH_COMMAND'])

class Repo(BaseRepo):
    GitCommandWrapperType = Git

With this change, the above example will allow the developer to use
Repo.clone_from(...) with the intended outcome. Otherwise the
developer will have two differing result when using Repo(...) vs
Repo.clone_from(...).

[mmulich]

I believe this could help address issues around confusing usage of an SSH key when using Repo.clone_from. For example, #400 #725 #866 and #916 to name a few. Or at least this may be a step towards it. The use of Repo.GitCommandWrapperType as stated in my example would need to be documented, but first things first... fix the implementation.

Thanks for having simple contribution instructions.

[Byron]

Thanks a lot for the fix for this long-standing oversight, it's much appreciated!

And thanks for the hint, I looked at the contribution guidelines and found it outdated, that's fixed now to hopefully further improve the experience.