> For example if we write in the terminal “git checkout” and after we press Space, a dropdown will appear with all available branches.

Nice.

I wouldn't want to discourage an interesting project like this, but a word to the wise: using something oneself is one thing, but adoption of things like this depends on them supporting basically all common commands out of the box, plus being able to configure/script support for additional commands without recompilation.

I saw some mention of json, so perhaps you've already got the latter.