A GitHub action to check whether a merge group PR is up to date relative to its base branch

Often, GitHub workflows are configured to run the same jobs (e.g., tests) on both pull_request and merge_group events. But if a PR is based on the head of it base branch and the tests pass on the pull_request event, there should be no need to rerun the tests on the merge_group event.

This action can be used to determine whether a PR is based on the head of its base branch and thus whether expensive jobs can be skipped.

1. Add a job like the following to your GitHub workflow:

   check-up-to-dateness:
     outputs:
       is-up-to-date: ${{ steps.main.outputs.is-up-to-date }}
     runs-on: ubuntu-latest
     steps:
       - id: main
         uses: trailofbits/check-up-to-dateness@v1

2. For any job you want to skip when merging, add the following:

   my-job:
     needs: [check-up-to-dateness]
     if: needs.check-up-to-dateness.outputs.is-up-to-date != 'true'

3. Under "Settings" -> "Branches" -> "Branch protection rules" -> "Require status checks to pass before merging" -> "Status checks that are required", add the job that you created in step 1, e.g.:
   This final step ensures that if an error occurs in check-up-to-dateness, it will not cause a PR to be merged.