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

@@ -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.
@@ -178,9 +184,14 @@ jobs:
             -c user.name=gitbot -c user.email=git@bot \
             merge -Xsubtree=library subtree/library; then
           echo "MERGE_CONFLICTS=yes" >> $GITHUB_ENV
+          for d in $(cat ../submodule-heads | cut -f2 -d" ") ; do git reset HEAD $d ; done
+          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 -a -m "Merge from $NEXT_COMMIT_HASH with conflicts"
         else
           echo "MERGE_CONFLICTS=no" >> $GITHUB_ENV
+          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/
         fi
         git replace -d subtree/library
         git replace -d main~1