Add git tab completion

[certik]

Besides directory and file completion that works (although needs some improvements), the other completion that I use is git completion.

I think we can simply implement it in Rust for now in some clean way. Only later worry about how to make it so that users can contribute their own completions.

[Hofer-Julian]

One alternative would be to integrate with https://github.com/carapace-sh/carapace.
That way we would get a lot of completions. I also use that one with nushell.