Subtree update automation: update submodules

Update submodules to the exact version that upstream uses at the commit
that we are trying to merge towards. We never modify the contents of
submodules, so we don't need to make any attempt to merge changes and
instead can directly pick the right upstream version.

Author: tautschnig

.github/workflows/update-subtree.yml

@@ -110,6 +110,8 @@ jobs:
         fi
 
         git checkout ${NEXT_COMMIT_HASH}
+        git submodule update --init -- library/
+        git submodule status  -- library/ > ../submodule-heads
         /usr/bin/time -v ../splitsh-lite/splitsh-lite --progress --prefix=library --target subtree/library
         git checkout -b subtree/library subtree/library
 
@@ -187,6 +189,11 @@ jobs:
         NEW_SUBTREE_HEAD=$(git rev-parse subtree/library)
         echo "NEW_SUBTREE_HEAD=${NEW_SUBTREE_HEAD}" >> $GITHUB_ENV
 
+        git submodule foreach 'git fetch'
+        git submodule foreach 'git fetch; grep $sm_path $toplevel/../submodule-heads | cut -f2 -d" " | xargs git checkout'
+        git -c user.name=gitbot -c user.email=git@bot \
+          commit -m "Update submodules" library/
+
         sed -i "s/^channel = \"nightly-.*\"/channel = \"nightly-${NEXT_TOOLCHAIN_DATE}\"/" rust-toolchain.toml
         git -c user.name=gitbot -c user.email=git@bot \
           commit -m "Update toolchain to ${NEXT_TOOLCHAIN_DATE}" rust-toolchain.toml