ruff-*-changed-files fail on some branches: no merge base
See e.g. https://gitlab.tails.boum.org/tails/tails/-/jobs/140056 and https://gitlab.tails.boum.org/tails/tails/-/jobs/140057.
My hunch is that this is caused by the fact we are asking git's ...
operator to do stuff it can't possibly do with an incomplete history ("Fetching changes with git depth set to 50..."). If so, GIT_DEPTH: 0
should fix it (https://docs.gitlab.com/ee/user/project/repository/monorepos/index.html#shallow-cloning).