title: brew update failure while executing git checkout
body: <p>After an interrupted <code>brew update</code> because of a network issue, Homebrew appears to be broken:</p>
  
  <pre><code>$ brew update
  error: pathspec 'master' did not match any file(s) known to git.
  Error: Failure while executing: git checkout -q master 
  </code></pre>
  
  <p>Looking at the <code>/usr/local</code> Git repository, there's no remote repository.</p>
  
  <pre><code>$ cd `brew --prefix`
  $ git remote -v
  $
  </code></pre>