use pr head ref and not the merge ref

Author: microdev1

.github/workflows/build.yml

@@ -135,16 +135,16 @@ jobs:
         GITHUB_TOKEN: ${{ github.token }}
         EXCLUDE_COMMIT: ${{ github.event.after }}
       run: python3 -u ci_changes_per_commit.py
+    - name: Set head sha
+      if: github.event_name == 'pull_request'
+      run: echo "HEAD_SHA=$(git show -s --format=%s $GITHUB_SHA | grep -o -P "(?<=Merge ).*(?= into)")" >> $GITHUB_ENV
     - name: Set base sha
       if: github.event_name == 'pull_request'
       run: |
-        DEEPEN=$((DEPTH - $(git rev-list HEAD --count))) && if((DEEPEN > 0)); then git fetch --no-tags --recurse-submodules=no --deepen=$DEEPEN; fi
-        echo "BASE_SHA=$(git rev-list $GITHUB_SHA --skip=$((DEPTH + 1)) --max-count=1)" >> $GITHUB_ENV
+        git fetch --no-tags --no-recurse-submodules --depth=$((DEPTH + 1)) origin $HEAD_SHA
+        echo "BASE_SHA=$(git rev-list $HEAD_SHA --skip=$DEPTH --max-count=1)" >> $GITHUB_ENV
       env:
         DEPTH: ${{ steps.get-last-commit-with-checks.outputs.commit_depth || github.event.pull_request.commits }}
-    - name: Set head sha
-      if: github.event_name == 'pull_request'
-      run: echo "HEAD_SHA=$(git rev-list $GITHUB_SHA --skip=1 --max-count=1)" >> $GITHUB_ENV
     - name: Get changes
       id: get-changes
       if: github.event_name == 'pull_request'