warn about untracked changes

I have just wasted a week by following the Ubuntu Packaging Guide and not understanding that I must add new files to the repository before test building.

For new developers and users not familiar with bzr, it would be much appreciated if `bzr builddeb` would warn about untracked files and changes. At least for the debian/ repository.

For users who are learning or experimenting with building packages, it is extra confusing that when you edit other files in debian/ directory without committing them, those changes are not ignored.