forge-ci: set commit message from main

.forgejo/workflows/pages.yaml

@@ -25,11 +25,12 @@ jobs:
         run: ikiwiki --setup ikiwiki.setup
       - name: Website upload
         run: |
+          git log -1 --pretty=%B > commit.txt
           cd public
           date > generated.txt
           # Note: the following account information will not work on GHES
           git config user.name "forgejo-actions[bot]"
           git config user.email "dev@ayakael.net"
           git add .
-          git commit -m "Update"
+          git commit -F ../commit.txt
           git push