Try alternative recursive downloads.

Base on actions/checkout#520 (comment)

.github/workflows/debug.yaml

@@ -18,11 +18,16 @@ jobs:
         with:
           # Checkout pull request HEAD commit instead of merge commit.
           ref: ${{ github.event.pull_request.head.sha }}
-          # Get a coiuple of previous commits to trace back to the start of the PR.
+          # Get a couple of previous commits to trace back to the start of the PR.
           fetch-depth: 10
-      - name: Fetch main
+      - name: Fetch commits in-between base and HEAD
+        # While the ancestor commit between HEAD and the branch base isn't in the local tree
+        # fetch 10 more parent commits of the branch base and branch head.
+        # Source: https://github.com/actions/checkout/issues/520#issuecomment-996873827
         run: |
-          git fetch --deepen=10 origin main HEAD
+          while [ -z $( git merge-base ${{ github.base_ref }} HEAD ) ]; do
+            git fetch --deepen=10 origin ${{ github.base_ref }} HEAD;
+          done
       - name: List previous commits
         run: |
           git log --decorate=full --oneline