zsh-git-prompt: Init at 0.5

This is a plugin of sorts for your `.zshrc` that can add status
information about the current git repository to your prompt. By
default it uses a python script and is thus easy on the dependencies
and not really worth packaging. But there's also a Haskell
implementation. I set it up sometime back, but then it died when the
libgmp version it was compiled with disappeared in a garbage
collection. So I decided that the Haskell version of zsh-git-prompt
might be worth packaging after all.