ci: Simplify the workflow

.github/workflows/merge-upstream.yml

@@ -47,28 +47,20 @@ jobs:
         git config --add user.name 'github-actions[bot]'
         git config --add user.email '6270544+github-actions[bot]@users.noreply.github.com'
 
-    - name: Retrieve the merge base
-      id: merge-base
+    - name: Merge the upstream
       run: |
-        rev=$(git merge-base upstream/${{ matrix.upstream_branch }} ${{ matrix.base_ref }})
-        echo "rev=$rev" > "$GITHUB_OUTPUT"
+        git merge --strategy ort -X hours upstream/${{ matrix.upstream_branch }}
 
     - name: Filter commits
       run: |
-        git switch -C upstream-recipes upstream/${{ matrix.upstream_branch }}
         git-filter-repo \
-          --path recipes/ --refs "${{ steps.merge-base.outputs.rev }}..HEAD" \
+          --path recipes/ --refs "${{ steps.start.outputs.rev }}..HEAD" \
           --commit-callback '
             commit.committer_name = commit.author_name
             commit.committer_email = commit.author_email
             commit.committer_date = commit.author_date
           ' --force
 
-    - name: Cherry-pick the filtered commits
-      run: |
-        git switch ${{ matrix.base_ref }}
-        git cherry-pick --strategy ort -X hours ..upstream-recipes
-
     - name: Create a pull request
       uses: peter-evans/create-pull-request@v5
       with: