Pass Git SHA/hash as a private cookiecutter variable

[ESKYoung]

cruft already stores the commit SHA/hash whenever in the .cruft.json file.

Whilst not a direct feature enhancement for cruft itself, it might be useful for users to be able to access to the commit hash in their downstream projects.

It should be straightforward to pass this as a private cookiecutter variable, since this is already done for the template variable (stored as the private variable _template).

This would solve a related ask in the cookiecutter project.

[ESKYoung]

I'm already working on this!