title: How does git commit --amend work, exactly?
body: <p>I have seen <em><a href="https://stackoverflow.com/questions/25948049/git-commit-amend-in-detached-head-state/25948372#25948372">&#39;git commit --amend&#39; in detached HEAD state</a></em>. The question requires the answer to be more complex than needs be.</p>
  <p>I'd like to understand just how <code>git commit --amend</code> works in a normal HEAD situation.</p>