Is fetch-depth 0 the only solution to resolve ${base.sha}...${head.sha}

[CircleCode]

I want to list files updated by the dev in its pull request (not the diff between base and head, but which files the developer explicitely touched).
The only viable solution I found was git diff --name-only --diff-filter=ACMRTUXB ${{ github.event.pull_request.base.sha }}...${{ github.event.pull_request.head.sha }} (note the 3 dots), but this require all the commits from common ancestor to be fetched.
Ideally, I would ask for commits of the 2 branches (base.ref and head.ref), but I do not find a way to specify 2 branches.

Am i missing something, or is it definitely not possible?