Async git prompt

[dandavison]

A module for caching and asynchronously computing git status info to display in a nushell prompt. This can be useful in large git repos where it is slow to query for staged/unstaged changes.

There is currently no automatic cache invalidation: you refresh the cache manually whenever you suspect it might be stale. It also doesn't display untracked files currently.

[fdncred]

This is cool. Would you mind moving this under the nu_scripts/prompt folder? Just to keep all the prompt stuff together?

[dandavison]

Would you mind moving this under the nu_scripts/prompt folder?

Done.

[fdncred]

thanks

[dandavison]

No problem, thanks for looking at this. It might be helpful to add an indicator showing when the prompt is displaying cached results (to remind you that it won't be changing unless you refresh the cache!). I'll probably open a PR for that in a bit. And maybe I'll start a missing features list in the README (cache invalidation, untracked files).