title: Bitbucket unable to merge unrelated branches
body: <p>I have repository (lets say A) and forked repository A-dev on Bitbucket. Everything had been worked good for 3 months. But recently, when I tried to create pull request in A-dev it says:</p>
  <p>Unable to merge<br />
  Unrelated branches</p>
  <p>Why this would happen and how it can be solved? How can I troubleshoot it?</p>
  <h2>Edit</h2>
  <p><img src="https://i.stack.imgur.com/ubWFw.png" alt="the screenshot of the error" /></p>