#!/usr/bin/env bash

git add --force _site
git commit -m "Release `date '+%Y-%m-%d %H:%M'`"
git subtree push --squash --prefix  _site/ origin gh-pages

#!/usr/bin/env bash

git add --force _site
git commit -m "Release `date '+%Y-%m-%d %H:%M'`"
git subtree split --prefix _site -b gh-pages
git push -f origin gh-pages:gh-pages

Change subtree push into split and force push
Change subtree push into split and force push