You can also use Git's own __git_ps1 from git-completion.bash in either bash or in zsh (see my other comment). Here's an example of it in use: http://github.com/avar/dotfiles/blob/master/.bashrc#L86

It doesn't give you info about whether you have staged files (but maybe it should). But it gives you a bunch of other neat stuff, like whether you're in a git-rebase or git-am operation