title: Unmerge a git branch, keeping post-merge commits
body: <p>Say there are two branches <code>master</code> and <code>branchA</code>. </p>
  
  <p>I work on <code>master</code>, a friend on <code>branchA</code>. 
  Once things are finalized, we merge <code>branchA</code> with <code>master</code>. </p>
  
  <p>After several commits on the merged <code>master</code>, the client wants the 
  work done on <code>branchA</code> removed, but <strong>NOT the commits done after the merge</strong>. </p>
  
  <p>How can we do it?</p>