Add shell completions for new git commands

[tony-sunny]

Fish version

3.0.2

Git 2.23.0 added new commands restore and switch which separates the duty for checkout command (https://github.blog/2019-08-16-highlights-from-git-2-23/#experimental-alternatives-for-git-checkout)

It'd be nice if someone can add shell completions for these two commands

[axionl]

I personally have a temporary solution to this problem and expect official support.

https://gist.github.com/axionl/1b038aeebd26dd8584558460204942fc

[zanchey]

Fixed with #6047 though completing the remaining options is left as an exercise.