tools: fix commit-lint GH Actions CI

PR-URL: #40845
Reviewed-By: James M Snell <jasnell@gmail.com>

.github/workflows/commit-lint.yml

@@ -9,14 +9,20 @@ 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=plusOne::$((${{ github.event.pull_request.commits }} + 1))"
+          echo "::set-output name=minusOne::$((${{ github.event.pull_request.commits }} - 1))"
       - uses: actions/checkout@v2
         with:
-          fetch-depth: 2
+          fetch-depth: ${{ steps.nb-of-commits.outputs.plusOne }}
+      - run: git reset HEAD^2
       - name: Install Node.js
         uses: actions/setup-node@v2
         with:
           node-version: ${{ env.NODE_VERSION }}
       - name: Validate commit message
         run: |
           echo "::add-matcher::.github/workflows/commit-lint-problem-matcher.json"
-          git rev-parse HEAD^ | xargs npx -q core-validate-commit --no-validate-metadata --tap
+          git rev-parse HEAD~${{ steps.nb-of-commits.outputs.minusOne }} | xargs npx -q core-validate-commit --no-validate-metadata --tap