title: Find a commit merging two ancestors
body: <p>If I have a git tree that looks like</p>
  
  <pre><code>A---o---o---o
               \
                C---o---o---o---D
               /
      B---o---o
  </code></pre>
  
  <p>How do I find C given A, B and (if it helps) D? I guess this is somehow the opposite of <code>git merge-base</code>: that finds (roughly) the "latest common ancestor" of two commits, here I'm looking for the "earliest common descendant".</p>
  
  <p>There could in theory be multiple candidates (e.g. if I added a commit directly merging A and B). But in this case there aren't, so I'm not worried about that situation.</p>