title: Git undo partial changes to a file
body: <p>How do I undo a part of the changes applied to a file that happened a while a go.
  I have the following commits and the accidental changes happened between <code>A</code> and <code>B</code> to 'file.txt'</p>
  
  <pre><code>...--A--B--...
  </code></pre>
  
  <p>I have a diff patch of the file in <code>file.txt-B-A.patch</code> which reverts <strong>all</strong> changes. 
  However I only want to undo <strong>certain changes</strong> in <code>file.txt</code>, much like manually picking changes in a merge conflict. Is there a way to do so without modifying the patch file?</p>