GitHub Actions中检查Pull Request变更文件的最优方案咨询

我正在配置GitHub Actions工作流，用于创建新Pull Request时对新增文件执行验证检查。受公司政策限制，只能使用已验证的GitHub Actions，所以计划依赖Git命令完成检查。

我已经配置了以下内容，用于获取最后一次提交与main分支的变更：

steps:
    - name: Checkout repository
      uses: actions/checkout@v2
      with:
        ref: ${{ github.event.pull_request.head.sha }}
        fetch-depth: 0

    - name: Run process and validate script
      id: changes
      run: |
       echo "git show --name-only --format=  HEAD"

我想运行git diff main --name-only，但在GitHub Action中执行时出现错误：

fatal: ambiguous argument 'main': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
Error: Process completed with exit code 128.

这个命令在本地能正常运行，因为本地有main分支，但GitHub Action里只拉取了PR分支，没有拉取main分支。我可以拉取main分支，但觉得这不是理想方案，示例配置如下：

steps:
    - name: Checkout PR branch
      uses: actions/checkout@v2
      with:
        ref: ${{ github.event.pull_request.head.sha }}
        fetch-depth: 0  # Fetch all history for branches and tags

    - name: Fetch main branch
      run: |
        git fetch origin main:main  # Fetch main branch as a local branch called main

    - name: Run process and validate script
      run: |
        # Now you can successfully run git diff between the PR branch and main
        git diff main --name-only

请问有没有更优的方案？

不需要单独拉取main分支到本地，有两种更高效的方式：

1. 直接使用远程main分支的引用 #

既然已经设置了fetch-depth: 0拉取了所有仓库历史，直接用origin/main作为对比目标即可，无需创建本地main分支：

steps:
    - name: Checkout repository
      uses: actions/checkout@v2
      with:
        ref: ${{ github.event.pull_request.head.sha }}
        fetch-depth: 0

    - name: Run process and validate script
      run: |
        git diff origin/main --name-only

2. 利用PR的base SHA（更精准通用） #

在Pull Request事件中，GitHub会提供github.event.pull_request.base.sha变量，它指向PR要合并到的目标分支（这里是main）的最新提交SHA。直接用这个SHA做对比，不需要依赖分支名，就算仓库默认分支不是main也能通用：

steps:
    - name: Checkout repository
      uses: actions/checkout@v2
      with:
        ref: ${{ github.event.pull_request.head.sha }}
        fetch-depth: 0

    - name: Run process and validate script
      run: |
        git diff ${{ github.event.pull_request.base.sha }} --name-only

这两种方式都省去了额外的git fetch步骤，直接利用已拉取的仓库内容完成对比，更简洁高效。

内容的提问来源于stack exchange，提问作者Marcusnh