git ubuntu should detect active VCS usage by source packages

> Separately, if we can programmatically detect this situation (packaging in
> Ubuntu maintained in bzr, vs. Debian), then we could add something both
> to "git ubuntu lint" and "git ubuntu clone" that detects this and warns
> you. I encourage a bug report about this.

I would suggest that a Vcs-* field in debian/control, plus an Ubuntu delta,
should be taken as an indicator to use that VCS instead of the git-ubuntu
branches.

(If the Vcs-* field is inherited from Debian and for this reason points to a
Vcs we don't want to use, well, we should just change that field as part of
the delta.)