title: git log the difference between 1 branch from another
body: <p>I have 2 branches A and B.</p>
  
  <p>Whenever I run a build, Branch A gets merged into Branch B. I want to be able to email out all the updates made in A, since the last time the build was ran.  How can I use <code>git log</code> to be able to copy all the commits made in A since the last A -> B merge?</p>