title: How to move a branch backwards in git?
body: <p>The title is not very clear. What I actually need to do often is the following:</p>
  
  <p>Let's say I have a development going on with several commits c1,c2,... and 3 branches A,B,C</p>
  
  <pre><code>c1--c2--c3--(B)--c4--(A,C)
  </code></pre>
  
  <p>Branch A and C are at the same commit.</p>
  
  <p>Now I want branch A to go back where B is, so that it looks like this:</p>
  
  <pre><code>c1--c2--c3--(A,B)--c4--(C)
  </code></pre>
  
  <p>Important is that this has to happen locally and on GitHub.</p>