add boolean property: 'files.dirty' if working dir is dirty

Tested with both JGit and native git. Handles both staged and unstaged commits.
Ahh, I missed that.

 1) Doesn't `waitFor()` wait for the process to exit? So you may not have to sit around reading all of the input in, but `git` still has to run to completion
 2) This won't work on Windows systems