commitlint job fails for MRs which are not directly branched from current main

In MRs we use CI_MERGE_REQUEST_DIFF_BASE_SHA in the --from option for commitlint. When this base SHA is not the current tip of development, commitlint fails silently which is confusing:

See, e.g. https://gitlab.developers.cam.ac.uk/uis/devops/experiments/rjw57/gitlab-utils/-/jobs/1763888

A workaround is to make sure the MR is based on the current main branch but it is non-obvious that this is a workaround.