title: How to delete the old git history?
body: <p>I have git repository with many, many (2000+) commits, for example:</p>
  
  <pre><code>                 l-- m -- n   
                  /
  a -- b -- c -- d -- e -- f -- g -- h -- i -- j -- k
                       \
                        x -- y -- z
  </code></pre>
  
  <p>and I want to truncate old log history - delete all commits from log history starting from (for example) commit "f" but as the beginning of repository.</p>
  
  <p>How to do it?</p>