workflow: first check if merge is necessary

.gitea/workflows/update-flake.yaml

@@ -23,11 +23,26 @@ jobs:
           git config user.email "actions@gitea.local"
 
       - name: Merge main branch
+        shell: bash
         run: |
-          git merge --squash origin/main
-          # solve merge conflict by taking flake.lock from main
-          git checkout origin/main -- flake.lock
-          git commit -m "Merge main branch squashed $(date -I)"
+            commits_ahead=$(git rev-list --count HEAD..origin/main)
+            echo "Commits ahead: $commits_ahead"
+            git log --oneline -5
+            echo "----------"
+            git log --oneline -5 origin/main
+
+            if [ $commits_ahead -ne 0 ]
+            then
+              git fetch origin
+              git merge --squash origin/main
+              git push --force-with-lease
+
+              # solve potential merge conflict by taking flake.lock from main
+              git checkout origin/main -- flake.lock
+              git commit -m "Merge main branch squashed $(date -I)"
+            else
+              echo "Merge not necessary"
+            fi
 
       - name: Set up Nix
         uses: cachix/install-nix-action@v31
@@ -39,7 +54,7 @@ jobs:
         shell: bash
         run: |
           git add flake.lock
-          if git diff --cached --quiet; then
+          if git diff --quiet; then
             echo "No changes to commit."
           else
             git commit -m "Update flake.lock $(date -I)"