Include original tarballs in the history

Also quit doing incremental tests; they were
subtly broken (can't update the master ref),
and now outright broken (since we do try to
update the orig master ref).
Use the tarball mtime for orig commits
Export source packages from git-deb commits
Don't unpack the same archive twice
Simplify the upstream history
Simplify history on the debian side
Include dsc data in upload tags
Split out signature checking
Rebuild historical dscs using upload tags