I know it's not a real answer...
I had some strange issues with sourcing git-prompt.sh in my .bashrc so I started to look for other solution. This one: http://www.jqno.nl/post/2012/04/02/howto-display-the-current-git-branch-in-your-prompt/ doesn't use __git_ps1 and author claims it works also on Mac (for now it works perfectly on my Ubuntu and it's easy to tweak).
I hope it helps!