Subtree update automation: create merge PR even when subtree/library is up-to-date

When the PR updating the `subtree/library` branch has been merged
already we may still need to bring updates into `main` that haven't
previously been added to a pull request. This situation arises when an
older merge-into-main PR was still open at the time where the subtree
automation creating the `subtree/library` update PR ran.

The changes in this PR avoid this problem by making sure the
merge-into-main part of the automation is run even when
`subtree/library` is already up-to-date.

Author: tautschnig

.github/workflows/update-subtree.yml

@@ -126,14 +126,14 @@ jobs:
         UPSTREAM_HEAD=$(git log --format=%H -n 1 rust-filtered/subtree/library)
         if [ "${UPSTREAM_HEAD}" = "${UPSTREAM_FROM}" ]; then
           echo "Nothing to do, ${UPSTREAM_FROM} matches ${UPSTREAM_HEAD} (${SUBTREE_HEAD_MSG})"
-          echo "MERGE_CONFLICTS=noop" >> $GITHUB_ENV
+          echo "SUBTREE_PR_REQUIRED=no" >> $GITHUB_ENV
         else
           git branch --set-upstream-to=origin/subtree/library
-          echo "MERGE_CONFLICTS=maybe" >> $GITHUB_ENV
+          echo "SUBTREE_PR_REQUIRED=yes" >> $GITHUB_ENV
         fi
 
     - name: Create Pull Request to update subtree/library
-      if: ${{ env.MERGE_CONFLICTS != 'noop' && env.SUBTREE_PR_EXISTS == 'no' }}
+      if: ${{ env.SUBTREE_PR_REQUIRED == 'yes' && env.SUBTREE_PR_EXISTS == 'no' }}
       uses: peter-evans/create-pull-request@v7
       with:
         title: 'Update subtree/library to ${{ env.NEXT_TOOLCHAIN_DATE }}'
@@ -151,7 +151,7 @@ jobs:
         path: verify-rust-std
 
     - name: Merge subtree/library changes
-      if: ${{ env.MERGE_CONFLICTS != 'noop' && env.MERGE_PR_EXISTS == 'no' }}
+      if: ${{ env.CURRENT_TOOLCHAIN_DATE != env.NEXT_TOOLCHAIN_DATE && env.MERGE_PR_EXISTS == 'no' }}
       run: |
         cd verify-rust-std
         # create-pull-request resets branches locally, implying that