command line arguments when massaging the dpkg-buildpackage arguments,
so that we don't end up giving dpkg-buildpackage contradictory
instructions.
+ * Cope with new git-buildpackage which provides gbp, rather than the
+ eponymous command, on PATH.
Configurability:
* Honour dgit-distros.DISTRO.cmd-CMD and .opts-CMD. Closes:#793427.