github actions: validate-kernel-commits: Add interdiff

Add steps to look for differences between upsteam commits referenced
in PR commit and the upsteam change they are backporting.  This is
accomplished with a customized version of interdiff with fuzzy
diffing and the run_interdiff.py helper script.  Since the custom
fuzzy diffing changes aren't available in upstream patchutils yet
this workflow pulls down and builds the custom version.

Author: bmastbergen

.github/workflows/validate-kernel-commits.yml

@@ -52,3 +52,41 @@ jobs:
           gh pr comment ${{ github.event.pull_request.number }} \
             --body "$(cat result.txt)" \
             --repo ${{ github.repository }}
+
+      - name: Install build dependencies for patchutils
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y build-essential autoconf automake libtool gnulib
+
+      - name: Clone and build custom patchutils
+        run: |
+          git clone https://github.com/kerneltoast/patchutils
+          cd patchutils
+          ./bootstrap
+          ./configure
+          make -j$(nproc)
+
+      - name: Download run_interdiff.py
+        run: |
+          curl -sL \
+            https://raw.githubusercontent.com/ctrliq/kernel-src-tree-tools/mainline/run_interdiff.py \
+            -o run_interdiff.py
+          chmod +x run_interdiff.py
+
+      - name: Run interdiff check
+        id: interdiff
+        run: |
+          python3 run_interdiff.py --repo . --pr_branch "${{ github.head_ref }}" --base_branch "${{ github.base_ref }}" --markdown --interdiff ./patchutils/src/interdiff | tee interdiff_result.txt
+          # Save non-empty results for PR comment
+          if grep -q -v "All backported commits match their upstream counterparts." interdiff_result.txt; then
+            echo "has_differences=true" >> $GITHUB_OUTPUT
+          fi
+
+      - name: Comment on PR if interdiff differences found
+        if: steps.interdiff.outputs.has_differences == 'true'
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          gh pr comment ${{ github.event.pull_request.number }} \
+            --body "$(cat interdiff_result.txt)" \
+            --repo ${{ github.repository }}