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

Tested with both JGit and native git. Handles both staged and unstaged commits.
That's not the speed up change. The speed up change is to not collect the entire output into a string and then check `isEmpty()` - way too costly. Instead as I described above, we return once there's a line of output. See https://github.com/ktoso/maven-git-commit-id-plugin/blob/master/src/main/java/pl/project13/maven/git/NativeGitProvider.java#L363