Github Action Heroku Git Deploy

deploy.yaml

# https://github.com/marketplace/actions/checkout
# https://help.github.com/en/actions/automating-your-workflow-with-github-actions/events-that-trigger-workflows#issue-comment-event-issue_comment
# https://devcenter.heroku.com/changelog-items/775
# https://github.com/actions/checkout#fetch-all-history-for-all-tags-and-branches
# https://github.com/cirrus-actions/rebase#installation
# https://github.com/actions/heroku/issues/10
---
name: Deploy to HEROKU_APP_NAME

on:
  issue_comment:
    types: [created]
jobs:
  deploy:
    name: deploy
    if: github.event.issue.pull_request != '' && contains(github.event.comment.body, '/deploy')
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - run: |
          git fetch --prune --unshallow
      - name: Deploy to Heroku
        env:
          HEROKU_APP_NAME: "HEROKU_APP_NAME"
          HEROKU_API_KEY: ${{ secrets.HEROKU_API_KEY }}
          HEROKU_LOGIN: ${{ secrets.HEROKU_LOGIN }}
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
        run: |
          cat > ~/.netrc << EOF
          machine api.heroku.com
            login $HEROKU_LOGIN
            password $HEROKU_API_KEY
          machine git.heroku.com
            login $HEROKU_LOGIN
            password $HEROKU_API_KEY
          EOF
          PR_NUMBER=$(jq -r ".issue.number" "$GITHUB_EVENT_PATH")
          URI=https://api.github.com
          API_HEADER="Accept: application/vnd.github.v3+json"
          AUTH_HEADER="Authorization: token $GITHUB_TOKEN"
          pr_resp=$(curl -s -H "${AUTH_HEADER}" -H "${API_HEADER}" \
                             "${URI}/repos/$GITHUB_REPOSITORY/pulls/$PR_NUMBER")
          HEAD_BRANCH=$(echo "$pr_resp" | jq -r .head.ref)
          echo "HEAD_BRANCH is $HEAD_BRANCH"
          git push --force https://git.heroku.com/$HEROKU_APP_NAME.git \
            "origin/${HEAD_BRANCH}:master"

with latest updates to git, line 46 should now be "origin/${HEAD_BRANCH}:refs/heads/master"