Record current git revision in www/head

type: documentation

.github/workflows/build.yaml

@@ -73,9 +73,12 @@ jobs:
         mdbook-version: latest
     - name: Build Book
       run: mdbook build book --dest-dir ../www/book
-    - name: Deploy Book
+    - name: Record Git Revision
       if: github.ref == 'refs/heads/master'
+      run: git rev-parse --verify HEAD > www/head
+    - name: Deploy Pages
       uses: peaceiris/actions-gh-pages@v3
+      if: github.ref == 'refs/heads/master'
       with:
         github_token: ${{secrets.GITHUB_TOKEN}}
         publish_branch: gh-pages