• check out this git repo $somewhere
• go to your systemd git ln -s $somewhere/[a-z]* .
• call eg ./bb

Base:System moves on. To include the changes rebase on it

create a worktree systemd_spec-base_system based on the base_system branch in the current directory

Run

./update_base_system

Check out a new branch with your changes and rebase. Important: the master branch tracks only merges. This avoids force pushes. The real content is in the second parent.

git checkout -b base_system-xxx-mine master^2
git rebase base_system

When happy update master and push stuff

git update-ref refs/heads/master $(git commit-tree HEAD^{tree} -p master -p HEAD -m "merge $(git branch --show-current)")
git push base_system-xxx-mine
git push origin base_system
git push origin master