Subtree update automation: update submodules (#394)

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.

See
https://github.com/tautschnig/verify-rust-std/actions/runs/15846078305/job/44668465448
for a successful run and
tautschnig#25 for the ensuing PR
(which, notably, includes [a commit that bumps
submodules](tautschnig@54d1d40)).

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.

Author: tautschnig

.github/workflows/update-subtree.yml

@@ -85,6 +85,7 @@ jobs:
       if: ${{ env.SUBTREE_PR_EXISTS == 'no' }}
       run: |
         cd splitsh-lite
+        sudo apt-get update
         sudo apt-get install -y golang libgit2-dev
         # git2go upstream hasn't been updated to more recent versions of
         # libgit2, so using a fork that does stay up to date
@@ -110,6 +111,10 @@ jobs:
         fi
 
         git checkout ${NEXT_COMMIT_HASH}
+        # Collect submodule commits; note that submodules (intentionally!)
+        # aren't initialized, hence lines will be prefixed with "-" (see git
+        # submodule --help).
+        git submodule status  -- library/ | sed 's/^-//' > ../submodule-heads
         /usr/bin/time -v ../splitsh-lite/splitsh-lite --progress --prefix=library --target subtree/library
         git checkout -b subtree/library subtree/library
 
@@ -165,6 +170,7 @@ jobs:
           fi
         fi
         git checkout main
+        git submodule foreach 'git fetch'
 
         # Tell git about the correct merge base to use, which is the subtree
         # head that we last merged from.
@@ -178,6 +184,8 @@ jobs:
             -c user.name=gitbot -c user.email=git@bot \
             merge -Xsubtree=library subtree/library; then
           echo "MERGE_CONFLICTS=yes" >> $GITHUB_ENV
+          # Ignore submodule conflicts, those are dealt with below.
+          for d in $(cat ../submodule-heads | cut -f2 -d" ") ; do git reset HEAD $d ; done
           git -c user.name=gitbot -c user.email=git@bot commit -a -m "Merge from $NEXT_COMMIT_HASH with conflicts"
         else
           echo "MERGE_CONFLICTS=no" >> $GITHUB_ENV
@@ -187,6 +195,12 @@ jobs:
         NEW_SUBTREE_HEAD=$(git rev-parse subtree/library)
         echo "NEW_SUBTREE_HEAD=${NEW_SUBTREE_HEAD}" >> $GITHUB_ENV
 
+        # Set submodules to upstream versions
+        git submodule update --init
+        git submodule foreach 'grep $sm_path $toplevel/../submodule-heads | cut -f1 -d" " | xargs git checkout'
+        git -c user.name=gitbot -c user.email=git@bot \
+          commit -m "Update submodules" library/
+
         sed -i "s/^channel = \"nightly-.*\"/channel = \"nightly-${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