title: git: moving branch head
body: <p>I have two git branches, "A" and "B", and commits numbered 1 thru 8.  My history looks like this</p>
  
  <pre><code>1 -&gt; 2 -&gt; 3 -&gt; 4[A] -&gt; 5 -&gt; 6 -&gt; 7 -&gt; 8[B]
  </code></pre>
  
  <p>I want to change it so my history looks like this:</p>
  
  <pre><code>1 -&gt; 2 -&gt; 3 -&gt; 4 -&gt; 5 -&gt; 6[A] -&gt; 7 -&gt; 8[B]
  </code></pre>
  
  <p>That is, I want to move the head of branch A from commit 4 to commit 6.</p>
  
  <p>What commands do I use to do this?</p>