Update rosdistro only when necessary

.github/workflows/update.yml

@@ -1,7 +1,7 @@
 name: Update rosdistro
 on:
   schedule:
-    - cron: "0 12 * * FRI"
+    - cron: "0 12 * * *"
   workflow_dispatch:
 jobs:
   update:
@@ -15,7 +15,26 @@ jobs:
           git config user.email 'github-actions[bot]@users.noreply.github.com'
       - name: Update rosdistro
         run: |
-          nix flake update rosdistro --commit-lock-file
+          nix build .#rosdistro --out-link /tmp/rosdistro-old
+          nix flake update rosdistro
+          nix build .#rosdistro --out-link /tmp/rosdistro-new
+          grep -Rh nixos: /tmp/rosdistro-old > /tmp/rosdistro-old.txt
+          grep -Rh nixos: /tmp/rosdistro-new > /tmp/rosdistro-new.txt
+          git reset --hard
+          if ! git diff /tmp/rosdistro-old.txt /tmp/rosdistro-new.txt > /tmp/rosdistro-diff; then
+            # redo update with nice commit message
+            nix flake update rosdistro --commit-lock-file
+            GIT_EDITOR='sed -i -e "1crosdistro update"' git commit --amend
+            GIT_EDITOR='sed -i -e "\$R/tmp/rosdistro-diff"' git commit --amend
+            cat <<EOF > /tmp/.body
+            Diff:
+            ```diff
+            $(cat /tmp/rosdistro-diff)
+            ```
+          EOF
+          else
+            echo "No relevant changes"
+          fi
           git show --format='%b' --no-patch > /tmp/.body
       - name: Create Pull Request
         uses: peter-evans/create-pull-request@v7