title: Git: how to go back before merge
body: <p>Thanks for your help.
  I am really confused...</p>
  
  <p>I merged a pull request for a team mate.</p>
  
  <p>Now I wish to go back to the situation before the merge, because I have realized that bugs are introduced.
  The pull request was done by a friend on the 24, I did the merge on the 24.</p>
  
  <ul>
  <li>I need to go back to the situation of the 20.</li>
  <li>what will happen to the changes done by the team mate? Will I be able to see/use them again?</li>
  </ul>
  
  <p>Thanks!!!</p>
  
  <pre><code>commit ff1e9ecfa13c5050957b77bd5b5695d802415867
  Merge: 053c894 437e11c
  Author: me
  Date:   Tue Mar 24 09:06:11 2015 +0100
  
  Merged ' changes on xxx
  
  commit 437e11c23ae7da68f8bb3698557bdc2aa3d9a614
  Merge: a129401 adfd8c5
  Author: someone else
  Date:   Tue Mar 24 00:00:18 2015 +1000
  
  Merge pull request #17 from 
  
  xxx
  
  commit adfd8c5c30541095ee48ce64da14740f92594ec8
  Author: someone else
  Date:   Mon Mar 23 23:54:06 2015 +1000
  
  xx
  
  commit 053c89426460ffaa86873bed342bc38e24a13096
  Merge: 849ec63 35fdbc6
  Author: me
  Date:   Fri Mar 20 07:57:13 2015 +0100
  </code></pre>