title: Get original branch name in Github action push event to master
body: <p>I'm trying to set up a Github action push event to master (so, a merge). The problem is that for what I want to do, I need the name of the original pull request branch. The Github <strong>pull request</strong> event makes this easy by calling <code>${{ github.head_ref }}</code>, but this option doesn't exist on a <strong>push</strong> event. I originally thought <code>${{ github.head }}</code> would give me what I want, but it actually just gives me <code>refs/heads/master</code>, not the name of the original PR branch.</p>
  <p>In other words, say I opened a PR to the master branch with a branch named <code>foo</code>. In a push event, I want to be able to get <code>foo</code> for what I want to do in the job.</p>
  <p>Is this possible?</p>