Pre-commit hook check doesn't work when using GitHub's Update branch button to merge main into a branch

[sumau]

The pre-commit hook check doesn't work when using GitHub's Update branch button to merge main into a branch because the commit is missing the signed-off by DBT message. See https://github.com/uktrade/hwrt/pull/104 as an example.

Resolution could be linked to #231.

Work-around until issue resolved:

As suggested by @nicopicchio , pull the branch, add an empty commit and push back:

git commit --allow-empty -m "Empty commit to resolve pre-commit hook check error"

[ian-leggett]

GitHub's "Update branch" action generates a merge commit via the GitHub API rather than a local Git client, which means local commit hooks are never executed and the trailer is not added. Since the resulting commit is not associated with an already merged PR, it also falls outside the scope of the current exemption logic.

My proposal

Since we already exempt commits originating from merged PRs, we could apply a similar exemption to merge commits from main. If the merge commit's second parent resolves to origin/main, we can safely bypass the check.