Restore dpkg::chroot-directory functionality
If we call dpkg inside a chroot we have to ensure that the temporary directory we construct to call dpkg --recursive is inside the chroot and that we strip the path to the chroot from the directory name we pass to dpkg.
Note that the added test succeeds before and (hopefully) after as we can't really chroot here or fiddle with the needed settings as we are already setting up apt to work with a quasi-chroot. The test perhaps helps in ensuring we don't break it too much in the future though.
(Broken five years (and one day) ago this seems to have an immense user base at the moment, but it might in the future via mmdebstrap)
References: f4959924 Reported-By: Johannes 'josch' Schauer on IRC Tested-By: Johannes 'josch' Schauer