Color separation for dirty/clean working directory

[samvasko]

It would be nice to have different color or some added character do denote dirty working directory.

[djs]

Detecting a dirty working directory is the most resource intensive operation in a git prompt. This script is already slow enough that I don't normally use it. I'm thinking that reworking it to use libgit2 should solve the performance problem. Even on Linux I've found zsh prompts to be unbearably slow with this functionality.

That said, I agree the option should be there, off by default.

[piotr-cz]

Perhaps if the detection of GIT directory would happen as suggested in #1, performance gain could be utilized to check if the directory is dirty/clean.