Add subcommand `clone` for use without Git installed

[mlange-42]

Would allow users to clone and explore a repository without having Git installed.

[peso]

Maybe this should be wont-fix?
If git-igitt aims to be a view-only client then clone is not needed.

[mlange-42]

As a local Git installation is not required, maybe it would still be useful and justified to provide this as the one and only Git command? It would allow users to browse a "remote" repo on a machine without Git.

But well, thinking a bit further, this would probably also require fetch then to be complete. 🤔