Fix subtree automation for submodules when there were no changes

While the prior fixes made sure that submodule changes are picked up,
the attempt to commit would fail when there were _no_ changes. See
https://github.com/model-checking/verify-rust-std/actions/runs/15878565075/job/44772389751
for a failing run.

Now we will only attempt to commit when there are changes.

Author: tautschnig

.github/workflows/update-subtree.yml

@@ -198,8 +198,10 @@ jobs:
         # Set submodules to upstream versions
         git submodule update --init
         git submodule foreach 'grep $sm_path $toplevel/../submodule-heads | cut -f1 -d" " | xargs git checkout'
-        git -c user.name=gitbot -c user.email=git@bot \
-          commit -m "Update submodules" library/
+        if ! git diff --quiet; then
+          git -c user.name=gitbot -c user.email=git@bot \
+            commit -m "Update submodules" library/
+        fi
 
         sed -i "s/^channel = \"nightly-.*\"/channel = \"nightly-${NEXT_TOOLCHAIN_DATE}\"/" rust-toolchain.toml
         git -c user.name=gitbot -c user.email=git@bot \