Pick SHA of commit to cherry-pick from merge commit (#3167)

Previous logic was picking commits with committer's SHA. But instead we
should cherry-pick the merge commit itself.

TODO: if a PR is merged with "Rebase and Merge" strategy current logic
will pick only the last commit. We usually don't merged like this, yet
for rare accousions we should gather all of the merged commits.

Signed-off-by: Sidorov, Dmitry <[email protected]>

Original commit:
KhronosGroup/SPIRV-LLVM-Translator@e08ed1a316f080f

Author: MrSidims

llvm-spirv/.github/workflows/backport-to-branch.yml

@@ -77,34 +77,38 @@ jobs:
           git fetch origin ${{ env.TARGET }}:${{ env.TARGET }}
           git checkout -b backport/pr-${{ github.event.issue.number }}-to-${{ env.TARGET }} origin/${{ env.TARGET }}
 
-      - name: Get list of PR commits
+      - name: Get commit sha
         id: commits
         uses: actions/github-script@v7
         with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
           result-encoding: string
           script: |
-            const { data } = await github.rest.pulls.listCommits({
+            const pr = await github.rest.pulls.get({
               owner: context.repo.owner,
               repo: context.repo.repo,
               pull_number: context.issue.number
             });
-            return data.map(c => c.sha).join(' ');
+            # FIXME: handle PRs that are merged with "Rebase and Merge" strategy
+            const sha = pr.data.merge_commit_sha;
+            if (!sha) {
+              throw new Error(`No merge_commit_sha found.`);
+            }
+            return sha;
 
-      - name: Cherry-pick commits
+      - name: Cherry-pick commit
         id: cherry
         run: |
           conflict=false
-          for sha in ${{ steps.commits.outputs.result }}; do
-            echo "Cherry-picking $sha"
-            if git cherry-pick "$sha"; then
-              echo "$sha"
-            else
-              echo "Conflict on $sha"
-              conflict=true
-              echo "CONFLICT_SHA=$sha" >> $GITHUB_ENV
-              break
-            fi
-          done
+          SHA="${{ steps.merge_sha.outputs.result }}"
+          echo "Cherry-picking squash-merge commit $SHA"
+          if git cherry-pick "$SHA"; then
+            echo "Cherry-picked $SHA"
+          else
+            echo "Conflict on $SHA"
+            conflict=true
+            echo "CONFLICT_SHA=$SHA" >> $GITHUB_ENV
+          fi
           echo "CONFLICT=$conflict" >> $GITHUB_ENV
 
       - name: Notify conflict