ci: add "no important files changed" workflow (#322)

Add a GitHub Actions workflow that runs when a new PR is opened (that targets the `main` branch).
The workflow checks if the PR changes any files that would result in the tests being re-run (for those changed exercises).
If this is the case, the workflow will add a comment to make the maintainer aware of this.
The comment will also suggest the maintainer add the "[no important files changed]" magic marker to the merge commit when the changed files won't influence the test results.

tracks-files/.github/workflows/no-important-files-changed.yml

@@ -0,0 +1,67 @@
+name: No important files changed
+
+on:
+  pull_request:
+    types: [opened]
+    branches: [main]
+
+permissions:
+  pull-requests: write
+
+jobs:
+  no_important_files_changed:
+    name: No important files changed
+    runs-on: ubuntu-22.04
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4.1.1
+
+      - name: Check if important files changed
+        id: check
+        run: |
+          set -exo pipefail
+
+          # fetch a ref to the main branch so we can diff against it
+          git remote set-branches origin main
+          git fetch --depth 1 origin main
+
+          for changed_file in $(git diff --diff-filter=M --name-only origin/main); do
+            if ! echo "$changed_file" | grep --quiet --extended-regexp 'exercises/(practice|concept)' ; then
+              continue
+            fi
+            slug="$(echo "$changed_file" | sed --regexp-extended 's#exercises/[^/]+/([^/]+)/.*#\1#' )"
+            path_before_slug="$(echo "$changed_file" | sed --regexp-extended "s#(.*)/$slug/.*#\\1#" )"
+            path_after_slug="$( echo "$changed_file" | sed --regexp-extended "s#.*/$slug/(.*)#\\1#" )"
+
+            if ! [ -f "$path_before_slug/$slug/.meta/config.json" ]; then
+              # cannot determine if important files changed without .meta/config.json
+              continue
+            fi
+
+            #  returns 0 if the filter matches, 1 otherwise
+            #  | contains($path_after_slug)
+            if jq --exit-status \
+              --arg path_after_slug "$path_after_slug" \
+              '[.files.test, .files.invalidator, .files.editor] | flatten | index($path_after_slug)' \
+              "$path_before_slug/$slug/.meta/config.json" \
+              > /dev/null;
+            then
+              echo "important_files_changed=true" >> "$GITHUB_OUTPUT"
+              exit 0
+            fi
+          done
+
+          echo "important_files_changed=false" >> "$GITHUB_OUTPUT"
+
+      - name: Suggest to add [no important files changed]
+        if: steps.check.outputs.important_files_changed == 'true'
+        uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea
+        with:
+          script: |
+            const body = "This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.\n\nIf this PR does **not** affect the result of the test (or, for example, adds an edge case that is not worth rerunning all tests for), **please add the following to the merge-commit message** which will stops student's tests from re-running. Please copy-paste to avoid typos.\n```\n[no important files changed]\n```\n\n For more information, refer to the [documentation](https://exercism.org/docs/building/tracks#h-avoiding-triggering-unnecessary-test-runs). If you are unsure whether to add the message or not, please ping `@exercism/maintainers-admin` in a comment. Thank you!"
+            github.rest.issues.createComment({
+              issue_number: context.issue.number,
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              body: body
+            })