Question about fetch depth

I think this is just me not able to understand the full problem, if so please feel free to close the issue.

The easiest way I figured out to replicate my issue is via git diff, even if I have a program that does that + more logic.

This is what I want to execute:

git diff "origin/$GITHUB_HEAD_REF..origin/$GITHUB_BASE_REF

It is always empty, even if the checkout action has fetch-depth set to 0.

    - name: Checkout code
      uses: actions/checkout@v2
      with:
        fetch-depth: 0

I had to manually fetch the origin git fetch origin in order to get git diff working as I want. Is it all expected and I am just missing something huge?

Thanks a lot

Start by reproducing the reported git diff "origin/$GITHUB_HEAD_REF..origin/$GITHUB_BASE_REF" command with actions/checkout@v2 and fetch-depth: 0. Inspect which refs are available after checkout and compare them with the result after git fetch origin; done means documenting the expected behavior or identifying a focused change.