title: post a diff between svn branch and a tag on review board
body: <p>I want to post an SVN diff to the review board; the diff is generated between the branch HEAD and the base tag.</p>
  
  <p>I used this command to generate the diff file :</p>
  
  <p><code>svn diff https:/path/to/branch/head https:/path/to/tag</code></p>
  
  <p>note that </p>
  
  <ul>
  <li><p>i tried to use <code>rbt diff revision1:revision2</code> command to generate the diff. I have a problem that review board only accepts revision range within the branch commits only (not accepting revision from tags).</p></li>
  <li><p>i tried to diff using the <code>svn diff</code> command then upload the file using <code>rbt post --diff-filename</code> but the command returned with an error requiring a base directory; i added the base dir to to be the root using <code>rbt post --basedir https:/path/to/root</code> ; the review boards accept but shows the diff on the web page like a diff between <code>https:/path/to/root/branches/featureName/path/to/changed/files</code> and <code>https:/path/to/root/path/to/changed/files</code> without showing that the diff is between branch and a tag like <code>https:/path/to/root/tag/path/to/changed/files</code>.</p></li>
  </ul>
  
  <p>is there any way to do such job ?</p>