tools: limit format-cpp checkout history

Use the first parent of the PR merge commit as the formatting base,
allowing a depth-two checkout instead of fetching the full history.

Assisted-by: Codex
Signed-off-by: inoway46 <inoueyuya416@gmail.com>

Author: inoway46

.github/workflows/linters.yml

@@ -54,7 +54,8 @@ jobs:
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1  # v7.0.1
         with:
-          fetch-depth: 0
+          # The first parent of the PR merge commit is the base branch commit.
+          fetch-depth: 2
           persist-credentials: false
       - name: Use Node.js ${{ env.NODE_VERSION }}
         uses: actions/setup-node@820762786026740c76f36085b0efc47a31fe5020  # v7.0.0
@@ -72,7 +73,7 @@ jobs:
           # because it is irrelevant. We already check if the formatter produced
           # a diff in the next line.
           # Refs: https://github.com/nodejs/node/pull/42764
-          CLANG_FORMAT_START="$(git merge-base HEAD refs/remotes/origin/$GITHUB_BASE_REF)" \
+          CLANG_FORMAT_START=HEAD^ \
             make format-cpp || true
           git --no-pager diff --exit-code && EXIT_CODE="$?" || EXIT_CODE="$?"
           if [ "$EXIT_CODE" != "0" ]