tools: avoid fetch extra commits when validating commit messages

.github/workflows/commit-lint.yml

@@ -2,18 +2,23 @@ name: "Commit messages adheres to guidelines at https://goo.gl/p2fr5Q"
 
 on: [pull_request]
 
+env:
+  NODE_VERSION: 14.x
+
 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:
-          # Last 100 commits should be enough for a PR
-          fetch-depth: 100
-      - name: Use Node.js 12
-        uses: actions/setup-node@v1
+          fetch-depth: ${{ steps.nb-of-commits.outputs.nb }}
+      - name: Use Node.js ${{ env.NODE_VERSION }}
+        uses: actions/setup-node@v2
         with:
-          node-version: 12.x
+          node-version: ${{ env.NODE_VERSION }}
       - name: Validate commit messages
         run: |
           echo "::add-matcher::.github/workflows/commit-lint-problem-matcher.json"