submodules: support for reference repositories

[timblechmann]

git submodule add has a support for reference repositories to speed up cloning. it would be great, if gitpython would expose this via Submodule.add and Submodule.update

[Byron]

Thanks for your suggestion.

As GitPython is in maintenance mode, there will be no new features added by me. For now and as a workaround, it should be possible to use the git command-line directly, such as in repo.git.submodule('add', ...).