title: How to find previous merge commit
body: <p>How can one find the previous merge commit between two branches? </p>
  
  <p>I would like to see the changes in my master branch since the last time I have merged the release branch into the master branch. To see the changes in the release branch since the last branch, it is as easy as <code>git diff ...release</code></p>
  
  <p>But obviously <code>git diff release...</code> doesn't work because it also includes all the changes before the last merge. Thus I think I need the commit id of the last merge to pass it to <code>git diff</code></p>
  
  <pre><code>git log --reverse --ancestry-path `git merge-base HEAD release`.. \ 
          --format=format:%H|head -n1
  </code></pre>
  
  <p>seems to work and can be used with <code>git diff $(...)</code>, but it seems awfully complicated. Is there some easier solution?</p>
  
  <p><strong>Example</strong> </p>
  
  <pre><code>  I
   / \
  A1 B1
   \ |
  |  M   
  |  |
  A2 B2
  </code></pre>
  
  <p>Here <code>I</code> is the initial commit. <code>A[12]</code> are the release commits and <code>B[12]</code> are the master commits. <code>M</code> is the previous commit. In the example the changes between the last merge and master is just the changes introduces by B2. <code>git merge-base A2 B2</code> returns <code>A</code>1. And <code>git diff B2 A1</code> includes the changes of <code>B1</code>. Thus the question is how to find M in the general more complex case so that one can run <code>git diff M B2</code> without having to manually find <code>M</code>. </p>