title: git merge --rebase: how to find "former/previous sha1"
body: <p>Imagine I have branch <code>develop</code> and branch <code>feature</code></p>
  
  <p>On <code>feature</code> there's a commit <code>shaX</code></p>
  
  <p>Someone merged <code>develop</code> on <code>feature</code> with <code>--rebase</code> and pushed, thus the feature commit is <code>shaY</code> now and history has been rewritten.</p>
  
  <p>Currently, I only have <code>shaY</code>. But I'd like to recreate the original <code>feature</code> branch from <code>shaX</code>.</p>
  
  <p>How can I get <code>shaX</code> when I only know <code>shaY</code>?</p>
  
  <hr>
  
  <p>I can (if required) accept a solution done on the computer of the dev that made the bad merge, but would prefer a solution that could be done on any machine (if possible), for my own knowledge, and because that developer could be gone or his machine destroyed</p>