Change git prompt colors to reflect LoC delta rileyjshaw/.supermac

I'm about to go through a series of refactors, and want a quick visual
indication of whether I'm adding or removing lines of code overall.
This change shows whether I'm red or green from the upstream branch
and from HEAD.

Annotated screenshot of what the new prompt colors represent

It's a bit slow. I'd like to speed it up if this works out.