Make git status to be async like in hydro

[DoctorRainer]

It would be cool if we didn't have to wait for git status like https://github.com/jorgebucaran/hydro implemented it.

[edouard-lopez]

@DoctorRyner that will be a nice improvement. I don't work with massive repo myself, but I'm open to PR and discuss there.

My understanding is that @jorgebucaran computes the content of the git status in a subprocess (fish --private) triggered on fish_prompt rendering and sets the universal variable _hydro_git.

[phush0]

it is already async you have to install async plugin for fish and add:
set -g async_prompt_functions _pure_prompt_git in your config

[DoctorRainer]

@phush0 I'm also talking about this being out of the box