github worflows: speed up git checkout.

This patch add 'fetch-depth: 0' to 'checkout' plugin on Github Worflows
to slightly speed up verifications.

.github/workflows/readme.yml

@@ -9,6 +9,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3.1.0
+        with:
+          fetch-depth: 0
       - name: Run readme test
         run: |
           error=0