Add gitoxide backend

[Byron]

This PR is in preparation for actual integration with the parent repository and to keep track of what still needs to be done before that can happen.

Tasks

• unify variable writing
• make gitoxide feature independent and assure CI tests it separately, as well as 'testbox'
• implement dirty check with gix (when available)
• unify git tests

Review Notes

• gix currently overrides git2 so it's possible to test it in test-box. In practice, it shouldn't matter as users select either git2 or gix