title: Looking for a way to calculate the number of lines that have NOT changed between two commits
body: <p>I'm working on a project that's under an old copyright, and I'm trying to make a good case to drop the copyright, because the project has significantly changed over the years.</p>
  
  <p>(Don't worry, not stealing anything, I'm gathering a case to get the copyright holder to agree with me).</p>
  
  <p>So what would really help, is an accurate statistic of what hasn't changed between two versions of a project.</p>
  
  <p>I'm using git, and I'm having trouble finding a solution.. </p>