ci: fetch-depth 0 to fetch complete history

With `${{ github.event.pull_request.commits }}` we don't fetch enough
commits to diff against the merge base. We require `pull_request.commits + 1`
commits to be able do that.

An alternative is to just fetch the complete git history as we don't
expect this repository to grow to a point where this has a
significant impact on CI performance.

.github/workflows/shasum-summary.yml

@@ -11,7 +11,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
         with:
-          fetch-depth: ${{ github.event.pull_request.commits }} 
+          fetch-depth: 0
       - name: show changed files
         run: git diff --no-commit-id --name-only ${{ github.event.pull_request.base.sha }}..${{ github.event.pull_request.head.sha }}
       - name: run shasum-summary