Complete git-deborig package transition

The new Core.pm, and installing git-deborig.

I've debdiff'd all the binary packages, and the changed list of installed files is what I expect.

I've autopkgtest'd just the addition of Core.pm, and then separately the git-deborig transition, for the latter adding in my draft devscripts.deb which doesn't include git-deborig:

autopkgtest --copy=/home/swhitton/src/debian/devscripts_2.25.18_all.deb:/tmp/devscripts.deb --setup-commands="DEBIAN_FRONTEND=noninteractive apt-get -y --no-install-recommends install /tmp/devscripts.deb" -- schroot unstable-amd64-sbuild

Edits to git-deborig will come later, including dropping the extra Perl deps, by leveraging Core.pm, as discussed.