Switch debian/watch to using Git

Switch debian/watch to using Git

On balance, I think this is better.  Ian has advised me to ignore the
"last resort" wording in uscan(1), and I've decided that since the
Debian package will be built from Git, there's not much point in
detecting an upstream release where I've not properly updated the Git
repository.  And if I do get that wrong, it's good to have something
that will detect it.

This means that uscan now not only detects new upstream versions, but
can also construct the correct orig.tar.gz for Debian purposes.