I have been searching for a solution to this for a while and have not found quite what I need.
I have several Git Repositories in a folder on my Mac (OSX 10.6) and
There's a Python based program, uncommitted that sounds like it would do exactly what you want. There's no git support for it yet (just hg and Subversion), but you may be able to help the author implement git support in his app, or take his ideas as how to implement your stuff (he documents his finding method on the project page I linked to).