title: IDEA doesn't show all branches that could be merged
body: <p>I'd like to merge with Intellij IDEA (11.1.3) via the menu 'VCS -> Git -> Merge Changes'. But the desired branch (mergeRequest15) is missing. But why? Is it because it seems that this branch bases on another branch (mergeRequest14)?</p>
  
  <p><strong>gitk --all shows:</strong></p>
  
  <p><img src="https://i.stack.imgur.com/NG5Mq.jpg" alt="gitk --all">
  <strong>IDEA shows:</strong></p>
  
  <p><img src="https://i.stack.imgur.com/Rfknq.jpg" alt="IDEA merge branches"></p>