gh-35808: .github/workflows/doc-build.yml: Fix generation of CHANGES.html
    
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### 📚 Description

This new feature from #35652 was supposed to show the changed HTML
files,
but a last minute change there destroyed it.

Here we fix it.

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URL: #35808
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Matthias Köppe

.github/workflows/doc-build.yml

@@ -50,7 +50,7 @@ jobs:
           if [ ! -f worktree-image/.gitignore ]; then cp .gitignore worktree-image/; fi
           (cd worktree-image && git add -A && git commit --allow-empty -m "old" -a && git tag -f old && git reset --hard new && git reset old)
           # Keep track of changes to built HTML
-          (cd /sage/local/share/doc/sage/html/en && find . -name "*.html" | xargs sed -i '/class="sidebar-brand-text"/s/Sage [0-9a-z.]* /Sage dev /'; git init && (echo ".buildinfo"; echo ".inv") > .gitignore; git add -A && git commit --quiet -m "old")
+          new_version=$(cat src/VERSION.txt); (cd /sage/local/share/doc/sage/html/en && find . -name "*.html" | xargs sed -i '/class="sidebar-brand-text"/s/Sage [0-9a-z.]* /Sage '$new_version' /'; git init && (echo "*.svg binary"; echo "*.pdf binary") >> .gitattributes && (echo ".buildinfo"; echo '*.inv'; echo '.git*'; echo '*.svg'; echo '*.pdf'; echo '*.png'; echo 'searchindex.js') > .gitignore; git add -A && git commit --quiet -m "old")
 
       - name: Incremental build
         id: incremental
@@ -80,7 +80,10 @@ jobs:
         # incremental docbuild may introduce broken links (inter-file references) though build succeeds
         run: |
           set -ex
-          make doc-clean doc-uninstall sagelib-clean && git clean -fx src/sage && ./config.status && make doc-html
+          mv /sage/local/share/doc/sage/html/en/.git /sage/.git-doc
+          make doc-clean doc-uninstall sagelib-clean && git clean -fx src/sage
+          mkdir -p /sage/local/share/doc/sage/html/en/ && mv /sage/.git-doc /sage/local/share/doc/sage/html/en/.git
+          ./config.status && make doc-html
         working-directory: ./worktree-image
         env:
           MAKE: make -j2
@@ -94,7 +97,7 @@ jobs:
           mkdir -p ./docs
           # Create changelog
           echo '## Preview of CHANGES.html'
-          (cd /sage/local/share/doc/sage/html/en && find . -name "*.html" | xargs sed -i '/class="sidebar-brand-text"/s/Sage [0-9a-z.]* /Sage dev /'; git diff --name-only) | tee ./docs/CHANGES.txt
+          (cd /sage/local/share/doc/sage/html/en && git diff --name-only) | tee ./docs/CHANGES.txt
           (cd /sage/local/share/doc/sage/html/en && git diff; rm -rf .git) > ./docs/html.diff
           echo '## Preview of html.diff'; head -n 400 ./docs/html.diff
           (echo '<p><a href="html.diff">HTML diff</a>'; sed -E 's,(.*),<p><a href="\1">\1</a>,' ./docs/CHANGES.txt) > ./docs/CHANGES.html