title: How can I find the first commit of a branch?
body: <p>Assume the following graph:</p>
  
  <pre><code>A -- B -- C -- D -- E -- F
        \
         G -- H -- I
  </code></pre>
  
  <p>I would like to find <code>G</code> in order to be able to an interactive rebase to squash the commits (but still keep one commit on the topic branch) which will be reviewed and merged later. I don't want to just rebase the whole branch, because I want to keep the information that there was a branch and it was merged.</p>
  
  <p>(I know that I can look at the history and just use the SHA checksum of the commit, but I'm looking for a way to do it without manually digging up the information or counting how many commits were made and use ~ from HEAD with that number). </p>
  
  <p>Edit: clarification of what I want to achieve:</p>
  
  <p>I want to avoid this:</p>
  
  <pre><code>A -- B -- C -- D -- E -- F -- J
        \                      /
         G -- H -- I -- -- -- -
  </code></pre>
  
  <p>And have something like this instead:</p>
  
  <pre><code>A -- B -- C -- D -- E -- F -- J
        \                      /
         G' -- -- -- -- -- -- -
  </code></pre>
  
  <p>In other words, I want to squash the commits with an interactive rebase on the topic branch into one, but still keep the branch and use a regular merge to integrate the changes from the topic branch into the master.</p>