title: extract multiple directories using git-filter-branch
body: <p>I have a big repository which currently contains multiple projects in top level subfolders, say <code>/a</code>, <code>/b</code>, <code>/c</code>, and <code>/d</code>.</p>
  
  <p>Now I want to split up that repository into two different repositories: one containing <code>/a</code> and <code>/b</code> and the other containing <code>/c</code> and <code>/d</code>.</p>
  
  <p>I am aware of <code>git filter-branch --subdirectory-filter</code>, which is perfect for extracting a single directory, but it seems not to be able to extract multiple directories at once.</p>
  
  <p>I am also aware of <code>git filter-branch --prune-empty --tree-filter</code>, which would allow me to delete everything, but the two wanted directories. This feels not completely right, as I have to manually specify all toplevel directories that might exist.</p>
  
  <p>Is there a better way to extract two directories out of a big repository?</p>
  
  <p>PS: Of course any good solution using something other than <code>git filter-branch</code> is fine. ;)</p>