title: How I can remove an already merged branch and preserve date
body: <p>Suppose following git history with mainline A and already merged branch B.</p>
  
  <pre><code>                  b1     b2
               -----•------•---            B
              /                \
     •------•----•--------------•-------•  A
     a1     a2   a3             a4      a5   
  </code></pre>
  
  <p>a4 is a merge commit w/ commit message merge "merge B into A.</p>
  
  <p>Is it possible to to change the history into following.</p>
  
  <pre><code>                    b1'  b2'
     •-----•-----•-----•-----•-----•  A
     a1     a2   a3                a5  
  </code></pre>
  
  <p>edit:
  I don't want to change the commit date if possible.</p>