title: Complete list of github actions contexts
body: <p>While investigating how, within github actions, I can diff the &quot;base&quot; version of a specific file in a repository, with the Pull Request's (&quot;head&quot;) version of the file ... while investigating this, I found among various sources (for example, <a href="https://github.community/t/how-do-i-get-the-git-diff-compared-to-main/179296" rel="noreferrer">github.community</a>, and <em>code examples</em> in the <a href="https://github.com/actions/checkout/#readme" rel="noreferrer">github.com/actions/checkout README file</a>) ... I found that the following context variables are available:</p>
  <ul>
  <li><code>github.ref</code></li>
  <li><code>github.sha</code></li>
  <li><code>github.event.pull_request.head.ref</code></li>
  <li><code>github.event.pull_request.head.sha</code></li>
  <li><code>github.event.pull_request.base.ref</code></li>
  <li><code>github.event.pull_request.base.sha</code></li>
  </ul>
  <p>However, other than the first two (<code>github.ref</code> and <code>github.sha</code>) I <em>cannot find the other four in any of the github actions documentation</em>.</p>
  <p>My question is this:  <em><strong>Is there any place where the full list of available context variables is documented?</strong></em></p>
  <p>I have, for example, <a href="https://docs.github.com/en/actions/learn-github-actions/contexts#github-context" rel="noreferrer">found this</a>, but it only lists context variables one level down from the github context object.  I can't find documentation for the more deeply nested variables noted above.  Likely there are other context variables that may be very useful, but I can't seem to find a <em>complete</em> list, rather only those that happen to be mentioned and scatter about in various code examples.</p>