Demo correct commit SHA incl. github action merge commit

GitHub will create multiple SHAs for pull request workflows:
https://www.kenmuse.com/blog/the-many-shas-of-a-github-pull-request/

.github/workflows/test.yml

@@ -21,5 +21,5 @@ jobs:
         bundle exec rails test
         ruby -e "$(curl -s https://undercover-ci.com/uploader.rb)" -- \
                           --repo twitchy-tortoise/undercover-test \
-                          --commit $GITHUB_SHA \
+                          --commit {{ github.event.pull_request.head.sha || github.sha }} \
                           --lcov coverage/lcov/undercover-test.lcov