Test sparse checkout.

git init sparse-test
cd sparse-test
git remote add -f origin ssh://git@github.com/<user>/doc.git
git config core.sparsecheckout true
echo "/index.html" >> .git/info/sparse-checkout
git checkout -b gh-pages
git pull origin gh-pages
<modify index.html>
git add -A .
git commit -m "Test sparse checkout."
git push origin gh-pages