Note to our future selfs:

We dropped directories for this branch that are still in main branch,
but were not in the released tarball. To prevent errors like
  dpkg-source: error: cannot represent change to dropped_dir/foo.ext

It did cause merge conflicts. Such conflicts were resolved by skipping
the commits. Preventing a mess in the git commit log was done
by separating the various git merge attempts from the git push to
the git repository at GitLab (originally SourceForge).
