title: How to get last commit sha from pull_request in push action?
body: <p>In the context of <a href="/questions/tagged/github-actions" class="post-tag" title="show questions tagged &#39;github-actions&#39;" rel="tag">github-actions</a>, how do I get sha of the last commit in the PR before it was merged?</p>
  <p>I want to get this sha in the context of <code>push</code> workflow, i.e.</p>
  <pre class="lang-yaml prettyprint-override"><code>on:
    push:
      branches:
        - master
  </code></pre>
  <p>If push was not triggered by a PR merge, then I need a way to ignore the action.</p>