title: Why am I getting pathspec error when I run git filter-branch?
body: <p>I have a git repo stored in Bitbucket with the following top-level structure:</p>
  
  <ul>
  <li>README.md</li>
  <li>dir_a</li>
  <li>dir_b</li>
  <li>dir_c</li>
  </ul>
  
  <p>Dirs a, b and c have grown to the point where I really want to refactor them into their own repos. After some googling I found the following article from Atlassian: <a href="https://confluence.atlassian.com/bitbucket/split-a-repository-in-two-313464964.html" rel="nofollow noreferrer">https://confluence.atlassian.com/bitbucket/split-a-repository-in-two-313464964.html</a>.</p>
  
  <p>But when I get to step 7 and run the command:</p>
  
  <pre><code>git filter-branch --index-filter 'git rm --cached -r dir_b dir_c' -- --all
  </code></pre>
  
  <p>I see the error:</p>
  
  <pre><code>Rewrite 7e0f6b3a2e52696d3ca934021e60095099c0dfd4 (1/60) (0 seconds passed, remaining 0 predicted)    fatal: pathspec 'dir_b' did not match any files
  index filter failed: git rm --cached -r dir_b dir_c
  </code></pre>
  
  <p>What is the cause of this error? How can I split up the repo while maintaining the commit history?</p>