tools: only validate first commit message of a PR

PR-URL: https://github.com/nodejs/node/pull/40740
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

.github/workflows/commit-lint.yml

@@ -1,4 +1,4 @@
-name: "Commit messages adheres to guidelines at https://goo.gl/p2fr5Q"
+name: "First commit message adheres to guidelines at https://goo.gl/p2fr5Q"
 
 on: [pull_request]
 
@@ -9,17 +9,14 @@ jobs:
   lint-commit-message:
     runs-on: ubuntu-latest
     steps:
-      - name: Compute number of commits in the PR
-        id: nb-of-commits
-        run: echo "::set-output name=nb::$((${{ github.event.pull_request.commits }} + 1))"
       - uses: actions/checkout@v2
         with:
-          fetch-depth: ${{ steps.nb-of-commits.outputs.nb }}
+          fetch-depth: 2
       - name: Install Node.js
         uses: actions/setup-node@v2
         with:
           node-version: ${{ env.NODE_VERSION }}
-      - name: Validate commit messages
+      - name: Validate commit message
         run: |
           echo "::add-matcher::.github/workflows/commit-lint-problem-matcher.json"
-          git log --oneline -${{ github.event.pull_request.commits }} HEAD^ | awk '{ if ($2 != "fixup!" && $2 != "squash!") { print $1 } }' | xargs npx -q core-validate-commit --no-validate-metadata --tap
+          git rev-parse HEAD^ | xargs npx -q core-validate-commit --no-validate-metadata --tap