title: Get pull request attached to work item
body: <p>I would like to access information about all pull requests that are linked to a particular work item.</p>
  
  <p>The relations on a work item result (e.g. <a href="https://.../_apis/wit/workItems/12345" rel="noreferrer">https://.../_apis/wit/workItems/12345</a>?$expand=all) shows links to the pull requests in this form:</p>
  
  <p><code>vstfs:///Git/PullRequestId/...</code></p>
  
  <p>How can I turn that relationship URI into the canonical URL for the pull request using the VSTS REST API? (e.g. <code>https://.../_apis/git/repositories/.../pullRequests/1234</code>)</p>