title: Conditions - Differentiate pull request and pull request merge
body: <p>Let's say I have a pipeline <code>my-pipeline</code> which 1) Install dependencies 2) Test my application.</p>
  <p>I set up a build validation policy which triggers <code>my-pipeline</code> upon creation of a pull request.</p>
  <p>Once the pull request is approved and merged, <code>my-pipeline</code> is triggered again.</p>
  <h2>Questions</h2>
  <p>Is there a way to differentiate the creation of the PR and the merged of the PR within my <code>yaml</code> file, so I can use conditions to trigger some scripts on creation but not on merge. Something similar to <code>condition: and(succeeded(), startsWith(variables['build.reason'], 'PullRequest'))</code></p>
  <p>For instance, I would trigger <code>test</code> on PR creation but not after the merge</p>