title: arc diff --update always create new review
body: <p>I download code from Gitlab, then create a new branch, update some files and create a review. Then I add a file, execute <code>arc diff --allow-untracked</code>, it reports <code>Usage Exception: There are several revisions which match the working copy:</code>. I execute <code>arc diff --allow-untracked --update D75</code> to update the existing review, but it creates a new review. 
  <code>Linting...
      No lint engine configured for this project.
      Running unit tests...
      No unit test engine is configured for this project.
      SKIP STAGING  No staging area is configured for this repository.
      Updating commit message...
      Created a new Differential revision:
          Revision URI: http://codereview.domain.com/D76</code> </p>
  
  <p>I try many times but it always creates a new revision. How to use <code>arc diff --update</code>?</p>