title: git-p4 sync on different branch and repo
body: <p>I'm trying to mirror a perforce repository, and git-p4 performs well enough on the main branch. I simply have to call <code>git-p4 rebase</code>. I'm also trying to mirror a different branch, but that's not working out so easily. I thought I could just do a <code>git-p4 rebase --branch=p4/whatever //open/branches/whatever</code> but <code>rebase</code> appears to ignore those tags, and if I do a <code>sync</code> instead I get the error message: <code>fast-import failed: warning: Not updating refs/heads/p4/whatever (new tip 14f7e14e5079bb730363440901adee102e5f04c9 does not contain 0ca89b0333f408367e3676720b109a08cc40b3ae)</code></p>
  
  <p>My initial import was a sync with @all, why wouldn't it be able to update, or is there a different/better way to do this?</p>