ci: Apply commitlint for all new commits

Consider all commits between the target branch and current HEAD. Assume the target branch is main if the target branch is unknown, i.e., no MR.

Here's a bash script that you may find useful for reviewing, e.g., in case you want to try the logic on a local system.

#!/bin/bash

# Populated by GitLab CI, see:
# https://docs.gitlab.com/ee/ci/variables/predefined_variables.html
#
# Added example value here to test that the below works locally
CI_MERGE_REQUEST_TARGET_BRANCH_NAME=main

base_commit_id="$(git merge-base HEAD "$CI_MERGE_REQUEST_TARGET_BRANCH_NAME")"
commit_ids="$(git rev-list --no-merges HEAD ^"$base_commit_id")"
for commit_id in $commit_ids; do
        #
        # msg is the full git-commit message for the given commit ID.  In other
        # words, it should be a reasonable replacement of $CI_COMMIT_MESSAGE.
        #
        #  %B: raw body (unwrapped subject and body)
        # -n1: just the one commit please
        #
        msg="$(git log --format=%B -n1 "$commit_id")"
    echo "$msg" # this is what we would pipe to commitlint
done

Closes #23 (closed).