title: how to reduce server git repo size when delete a branch
body: <p>I have use gitlabhq configed a git server. 
  All Repos save in <code>/repos/</code> directory at server.</p>
  
  <p>I have a project, and have a branch, this branch have big files.
  For save disk space, i delete this branch use <code>git push origin :test</code>.</p>
  
  <p>But when i go to my git server, found this project repo size not change.</p>
  
  <p>So my questions is how to  reduce server git repo size when delete a branch?</p>
  
  <p>I have used git filter-branch --tree-filter 'rm -f bigfile.zip' HEAD, also seems can not
  reduce server git repo size.</p>