title: Get the name of the source branch from which the PR was merged
body: <p>I use Azure DevOps as my CI/CD platform and I am currently working on developing a YAML file that requires the name of the source branch from which the PR was merged.</p>
  <p>For example: If I open a PR from branch <code>feature</code> to branch <code>master</code>; then when I merge this PR; my pipeline gets triggered. In this trigger, I would like to get the source branch name which in my case is <code>feature</code>.</p>
  <p>I've looked into <code>System.PullRequest.SourceBranch</code> and also <code>Build.SourceBranch</code>; but these options are only useful if I either have a PR open or when to get the target branch name upon merging (at least in my case)</p>
  <p>Is there some documentation that I can refer to for getting the source branch name in the YAML when a PR is merged?</p>