publish-gh-pages: don't reset hard to have network bandwidth in transferring all files from scratch

publish-gh-pages

@@ -3,13 +3,11 @@
 set -eu
 git push
 ./build-doc --github-pages
-git branch -D gh-pages
-git checkout --orphan gh-pages
-git reset
+git checkout gh-pages
+git checkout master -- '*.png' _config.yml
 cp out/README.html index.html
-git add _config.yml index.html
+git add index.html
 git commit --message "$(git log -n1 --pretty='%H' master)"
 git push --force
-git symbolic-ref HEAD refs/heads/master
-git reset
+git checkout -
 rm index.html