Add gbp.conf and revert watch file back to the correct one

In the current Debian packaging repository format, this package is intended to import the upstream release tarballs which are much smaller than the upstream git contents. Revert the changes in debian/watch to pass correct instructions to uscan. Also document current practice in debian/gbp.conf and update debian/README.source to show what gbp settings are used and what command will do the new upstream version imports.

The attempted changes in debian/watch needs to be done by adopting the 'upstream-vcs-tag' setting in gbp.conf, and that can only be done once the Debian packaging repository has adopted DEP-14 branch naming.