title: Update git-svn git mirror without redownloading everything
body: <p>I made a git mirror of a svn repository using </p>
  
  <pre><code>    git svn clone svn://svn.svnrepo.com
  </code></pre>
  
  <p>I uploaded it to github and was able to update the local repo using</p>
  
  <pre><code>    git svn rebase svn://svn.svnrepo.com
  </code></pre>
  
  <p>and push the updates to github</p>
  
  <p>Later after I had deleted the local copy I wanted to update the remote again</p>
  
  <p>So I cloned the git repo</p>
  
  <pre><code>    git clone git://git.gitrepo.com
  </code></pre>
  
  <p>But when I tried to rebase I got an error</p>
  
  <pre><code>    git svn rebase svn://svn.svnrepo.com
  
      Data from a previous version of git-svn exists, but
  
      (required for this version (1.8.3.2) of git-svn) does not exist.
  
      Done migrating from a git-svn v1 layout
  </code></pre>
  
  <p>then it gives an Unable to determine upstream SVN information from HEAD history error</p>
  
  <p>Ive tried:</p>
  
  <pre><code>    git svn init svn://svn.svnrepo.com
  </code></pre>
  
  <p>with:</p>
  
  <pre><code>    git svn fetch
  </code></pre>
  
  <p>but that starts a re-download of the entire svn repo which takes over 24 hours
  The current svn revision is marked by the git-svn-id in the commit log and seems fine, how do I stop git svn fetch from re-fetching files that already exist or rebase it without an entire re-download? This mirror never had commits that would make it different from the svn upstream.</p>