title: Warning on executing "git rebase"
body: <p>I tried to modify the commit message of an old commit using the following command.</p>
  
  <pre><code>git rebase -i HEAD~3
  </code></pre>
  
  <p>After editing the commit list displayed in the text editor (replacing <code>pick</code> with <code>edit</code>), I got the following warning.</p>
  
  <pre><code>Warning: the SHA-1 is missing or isn't a commit in the following line
  </code></pre>
  
  <p>Then when execuing <code>git commit --amend</code>, I got the following error and could not modify the commit message:</p>
  
  <pre><code>fatal: Could not open file .git/rebase-merge/done for reading: No such file or directory
  </code></pre>
  
  <p>What should I do to avoid these errors?</p>