title: brew tap facebook/fb results in Fetching error
body: <p>I try to update Homebrew as usual. Recently when I issue the command <code>brew update</code>, the following error occurs:</p>
  <blockquote>
  <p>Error: Fetching /usr/local/Homebrew/Library/Taps/facebook/homebrew-fb failed!</p>
  </blockquote>
  <p>The Tap is associated with the software <code>Buck</code>, which I need for development.</p>
  <p>I further investigate the issue by following the <a href="https://buck.build/setup/getting_started.html" rel="noreferrer">installation instruction</a> in the Buck official website, I issue the command again:</p>
  <pre><code>% brew tap facebook/fb
  ==&gt; Unshallowing facebook/fb
  fatal: couldn't find remote ref refs/heads/master
  Error: Failure while executing; `git fetch --unshallow` exited with 128.
  </code></pre>
  <p>Apparently, the Git source has problems. I try to &quot;untap&quot; it:</p>
  <pre><code>% brew untap facebook/fb
  Error: Refusing to untap facebook/fb because it contains the following installed formulae or casks:
  buck
  </code></pre>
  <p>No luck. How can I resolve this problem?</p>