Add the template commit log to the PR description in updater workflow

.github/workflows/centralised_cruft_updater.yml

@@ -73,6 +73,10 @@ jobs:
       REPO_FULL: ${{ matrix.repo }}
       BOT: "SunPyBot"
       BRANCH: "cruft/update"
+      PR_BODY: |
+        This is an autogenerated PR, which applies the latest changes from the [SunPy Package Template](https://github.com/sunpy/package-template).
+
+        If this pull request has been opened as a draft there are conflicts which need fixing.
 
     steps:
       - name: Set safe repo name
@@ -142,6 +146,8 @@ jobs:
         env:
           EXTRA_CONTEXT: ${{ inputs.extra_context }}
         run: |
+          OLD_COMMIT=$(jq -r '.commit // empty' .cruft.json 2>/dev/null || echo "")
+
           EXTRA_ARGS=()
           if [ -n "$EXTRA_CONTEXT" ]; then
             EXTRA_ARGS=(--variables-to-update "$EXTRA_CONTEXT")
@@ -155,6 +161,10 @@ jobs:
             echo "merge_conflicts=0" >> "$GITHUB_OUTPUT"
           fi
 
+          NEW_COMMIT=$(jq -r '.commit // empty' .cruft.json 2>/dev/null || echo "")
+          echo "old_commit=${OLD_COMMIT}" >> "$GITHUB_OUTPUT"
+          echo "new_commit=${NEW_COMMIT}" >> "$GITHUB_OUTPUT"
+
       - name: Check if only .cruft.json is modified
         id: cruft_json
         run: |
@@ -179,6 +189,32 @@ jobs:
           fi
           echo "merge_conflicts=$MERGE_CONFLICTS" >> "$GITHUB_OUTPUT"
 
+      - name: Generate commit log
+        id: commit_log
+        if: steps.cruft_json.outputs.has_changes == '1'
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          OLD_COMMIT: ${{ steps.cruft_update.outputs.old_commit }}
+          NEW_COMMIT: ${{ steps.cruft_update.outputs.new_commit }}
+        run: |
+          COMMIT_LOG=""
+          if [ -n "$OLD_COMMIT" ] && [ -n "$NEW_COMMIT" ] && [ "$OLD_COMMIT" != "$NEW_COMMIT" ]; then
+            COMMIT_LOG=$(gh api "repos/${{ github.repository }}/compare/${OLD_COMMIT}...${NEW_COMMIT}" \
+              --jq '.commits[] | "- [" + .sha[0:7] + "](" + .html_url + ") " + (.commit.message | split("\n")[0])' 2>/dev/null || echo "")
+          fi
+
+          {
+            echo "$PR_BODY"
+            if [ -n "$COMMIT_LOG" ]; then
+              echo ""
+              echo "## Changes from the template"
+              echo ""
+              echo "$COMMIT_LOG"
+            fi
+          } > pr_body.md
+
+          echo "path=$(pwd)/pr_body.md" >> "$GITHUB_OUTPUT"
+
       # There is already an open PR, so its branch just needs a new commit.
       # create-pull-request is not used here because it resets the
       # branch to the base, which would discard any manual conflict fixes.
@@ -199,6 +235,15 @@ jobs:
           git commit -m "Run cruft update from package template"
           git push "https://${BOT}:${BOT_FORK_PAT}@github.com/${FORK_REPO}.git" "HEAD:${BRANCH}"
 
+      - name: Update existing PR description
+        if: steps.cruft_json.outputs.has_changes == '1' && steps.existing.outputs.existing_pr != '0'
+        env:
+          GH_TOKEN: ${{ secrets.BOT_PR_PAT }}
+          PR_NUMBER: ${{ steps.existing.outputs.existing_pr }}
+          BODY_PATH: ${{ steps.commit_log.outputs.path }}
+        run: |
+          gh pr edit "$PR_NUMBER" --repo "$REPO_FULL" --body-file "$BODY_PATH"
+
       # No open PR, so build the branch on the fork from scratch and open one.
       # maintainer-can-modify must be false as BOT_PR_PAT has no write access to
       # the fork; the allow-edits job turns it on afterwards.
@@ -218,12 +263,7 @@ jobs:
           branch: ${{ env.BRANCH }}
           delete-branch: true
           title: 'Updates from the package template'
-          body: |
-            This is an autogenerated PR, which applies the latest changes from the [SunPy Package Template](https://github.com/sunpy/package-template).
-
-            If this pull request has been opened as a draft there are conflicts which need fixing.
-
-            **To run the CI on this pull request you will need to close it and reopen it.**
+          body-path: ${{ steps.commit_log.outputs.path }}
           draft: ${{ steps.conflicts.outputs.merge_conflicts == '1' }}
 
       - name: Prepare status for collation