Optimise fetching all commits in a pull request

My goal is to lint all commit messages in a pull request, or in other words, since branching off the default branch. My workaround is

    - uses: actions/checkout@v2
      with:
        fetch-depth: 0
        ref: ${{ github.event.pull_request.head.sha }}
    # grab default branch to calculate branch point to know when to stop linting
    - if: ${{ github.base_ref }}
      run: git branch ${{ github.base_ref }} origin/${{ github.base_ref }}

but for large repos, it would be nice if this action could do the smart thing and only fetch what is needed. Thanks in advance.

[polarathene]

I don’t see anything in the pull_request body that tells me how many commits to fetch if you do a shallow clone

${{ github.event.pull_request.commits }} is available for that. I guess it was not a year ago.

[ericzolf]

My use case is different but the need is the same: I want to be able to list all files changed in the PR (and decide accordingly if a step needs to be run) but it can only work if the base_ref is also fetched, which currently, not knowing the number of commits required, can only be done by fetching the whole repository.