title: Hg merge -> revert -> merge
body: <p>I've stuck with a small Mercurial problem: I merged a branch A into a branch B and everything succeeded, but after that one of my teammates reverted the branch B back to pre-merge status.
  Now I need to merge branch A once again into branch B, but I get the following error:</p>
  
  <pre><code>abort: merging with a working directory ancestor has no effect
  </code></pre>
  
  <p>How to solve the problem?</p>