If nothing has changed, don't proceed.

ci-tools/update-nixpkgs

@@ -42,7 +42,10 @@ git checkout "${TARGET_BRANCH}" || git checkout -b "${TARGET_BRANCH}"
 nix-shell ./shell.nix --run update-nixpkgs
 
 # Show us what we did
-git diff
+if ! git diff --exit-code; then
+    echo "No changes."
+    exit 0
+fi
 
 # nix-build -A morph -o result-after
 # diff=$(nix --extra-experimental-features nix-command store diff-closures ./result-before/ ./result-after/)