title: How to use interactive rebase on the first (root) commit of a branch?
body: <p>If I'm in the following situation,</p>
  
  <pre><code>$ git log --oneline
  * abcdef commit #b
  * 123456 commit #a
  </code></pre>
  
  <p>I know I can always run</p>
  
  <pre><code>$ git reset HEAD~ 
  $ git commit --amend
  </code></pre>
  
  <p>However, I tried to run</p>
  
  <pre><code>$ git rebase -i HEAD~2
  </code></pre>
  
  <p>but I got</p>
  
  <pre><code>fatal: Needed a single revision
  invalid upstream HEAD~2
  </code></pre>
  
  <p>Hence my question: is there a way to use <code>git rebase</code> to squash these two commits or not?</p>