title: Update a single external repo in Bazel
body: <p>I want to update a single external dependency before I do <code>bazel build</code>. 
  Is there a way to do this? </p>
  
  <p><code>bazel sync</code> refreshes all the external dependencies, I am looking for something to just refresh a single external dependency. </p>
  
  <p><code>bazel fetch</code> does not seem to work for me, at least when I tried fetching a remote git repo.</p>