Subtree update automation: do not use git subtree merge --squash

`git subtree merge --squash` will always reset the subtree to the state
of the tree that is being merged from, which effectively eradicates all
our local changes. This was just masked by merge conflicts arising as we
were always attempted to merge from some long-ago version as we hadn't
consistently kept "git-subtree-split" markers.

This PR now amends the pull request descriptions to make sure we retain
the necessary information and uses `git merge` instead of the subtree
command.

Author: tautschnig

.github/workflows/update-subtree.yml

@@ -137,11 +137,14 @@ jobs:
       uses: peter-evans/create-pull-request@v7
       with:
         title: 'Update subtree/library to ${{ env.NEXT_TOOLCHAIN_DATE }}'
-        body: |
+        body: >
           This is an automated PR to update the subtree/library branch to the changes
-          from ${{ env.CURRENT_TOOLCHAIN_DATE }} (${{ env.CURRENT_COMMIT_HASH }})
-          to ${{ env.NEXT_TOOLCHAIN_DATE }} (${{ env.NEXT_COMMIT_HASH }}), inclusive.
-          **Do not merge this PR using the merge queue. Instead, use the rebase strategy.**
+          from ${{ env.CURRENT_TOOLCHAIN_DATE }} (rust-lang/rust@${{ env.CURRENT_COMMIT_HASH }})
+          to ${{ env.NEXT_TOOLCHAIN_DATE }} (rust-lang/rust@${{ env.NEXT_COMMIT_HASH }}), inclusive.
+
+          **Review this PR as usual, but do not merge this PR using the GitHub web interface.
+          Instead, once it is approved, use `git push` to literally push the changes to `subtree/library`
+          without any rebase or merge.**
         branch: update-subtree/library
         delete-branch: true
         base: subtree/library
@@ -156,15 +159,25 @@ jobs:
         fi
         git checkout main
 
+        # Tell git about the correct merge base to use, which is the subtree
+        # head that we last merged from.
+        PREV_SUBTREE_HEAD=$(git log --grep="^git-subtree-split:" | egrep '^[[:space:]]+git-subtree-split:' | awk '{print $2;exit}')
+        git replace --graft subtree/library ${PREV_SUBTREE_HEAD}
+        git replace --graft main ${PREV_SUBTREE_HEAD}
+
         # This command may fail, which will require human intervention.
         if ! git \
             -c user.name=gitbot -c user.email=git@bot \
-            subtree merge --prefix=library subtree/library --squash; then
+            merge -Xsubtree=library subtree/library; then
           echo "MERGE_CONFLICTS=yes" >> $GITHUB_ENV
           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" >> $GITHUB_ENV
         fi
+        git replace -d subtree/library
+        git replace -d main~1
+        NEW_SUBTREE_HEAD=$(git rev-parse subtree/library)
+        echo "NEW_SUBTREE_HEAD=${NEW_SUBTREE_HEAD}" >> $GITHUB_ENV
 
         sed -i "s/^channel = \"nightly-.*\"/channel = \"nightly-${NEXT_TOOLCHAIN_DATE}\"/" rust-toolchain.toml
         git -c user.name=gitbot -c user.email=git@bot \
@@ -178,11 +191,16 @@ jobs:
       uses: peter-evans/create-pull-request@v7
       with:
         title: 'Merge subtree update for toolchain nightly-${{ env.NEXT_TOOLCHAIN_DATE }}'
-        body: |
+        body: >
           This is an automated PR to merge library subtree updates
-          from ${{ env.CURRENT_TOOLCHAIN_DATE }} (${{ env.CURRENT_COMMIT_HASH }})
-          to ${{ env.NEXT_TOOLCHAIN_DATE }} (${{ env.NEXT_COMMIT_HASH }}), inclusive.
+          from ${{ env.CURRENT_TOOLCHAIN_DATE }} (rust-lang/rust@${{ env.CURRENT_COMMIT_HASH }})
+          to ${{ env.NEXT_TOOLCHAIN_DATE }} (rust-lang/rust@${{ env.NEXT_COMMIT_HASH }}), inclusive.
           This is a clean merge, no conflicts were detected.
+          **Do not remove or edit the following annotations:**
+
+          git-subtree-dir: library
+
+          git-subtree-split: ${{ env.NEW_SUBTREE_HEAD }}
         branch: sync-${{ env.NEXT_TOOLCHAIN_DATE }}
         delete-branch: true
         base: main
@@ -193,12 +211,17 @@ jobs:
       uses: peter-evans/create-pull-request@v7
       with:
         title: 'Merge subtree update for toolchain nightly-${{ env.NEXT_TOOLCHAIN_DATE }}'
-        body: |
+        body: >
           This is an automated PR to merge library subtree updates
-          from ${{ env.CURRENT_TOOLCHAIN_DATE }} (${{ env.CURRENT_COMMIT_HASH }})
-          to ${{ env.NEXT_TOOLCHAIN_DATE }} (${{ env.NEXT_COMMIT_HASH }}) (inclusive)
+          from ${{ env.CURRENT_TOOLCHAIN_DATE }} (rust-lang/rust@${{ env.CURRENT_COMMIT_HASH }})
+          to ${{ env.NEXT_TOOLCHAIN_DATE }} (rust-lang/rust@${{ env.NEXT_COMMIT_HASH }}) (inclusive)
           into main. `git merge` resulted in conflicts, which require manual resolution.
           Files were commited with merge conflict markers.
+          **Do not remove or edit the following annotations:**
+
+          git-subtree-dir: library
+
+          git-subtree-split: ${{ env.NEW_SUBTREE_HEAD }}
         branch: sync-${{ env.NEXT_TOOLCHAIN_DATE }}
         delete-branch: true
         base: main