title: git how to find commit hash where branch originated from
body: <p>Pretend I branched from my master branch to a topic branch, and then I did a few commits on my topic branch. Is there a command that tells me the commit hash on the master branch that my topic branch originated from?</p>
  
  <p>Ideally, I wouldn't have to know how many commits I've made ( trying to avoid HEAD^5 ).</p>
  
  <p>I've googled and SO'd around and can't seem to land on the answer. Thanks!</p>