title: how can I remove the unwanted objects from my repo after filter-branch --subdirectory-filter
body: <p>I'm using</p>
  
  <pre><code>git filter-branch --subdirectory-filter dir/name -- --all
  </code></pre>
  
  <p>to build a repo that only has history relating to that dir/name.  Before I do the filter, I clone the original repo (which is very much bigger) into a tmp dir.  After the filter-branch, the repo looks just how I want it, with one exception:  It seems to still contain all the objects from the original repo even though they're not shown in "git log."</p>
  
  <p>How can I remove all those unwanted objects completely?</p>
  
  <p>I've tried things like:</p>
  
  <pre><code>git reflog expire --expire=now --all
  git gc --aggressive --prune=now
  </code></pre>
  
  <p>It's clear to me that I don't know why they're still there or what it means to remove them, but I'd sure like to.  A bit of possibly related information working against me is that I had done a git repack -a on my source repo a while back and it seems to copy that packfile over to the new repo.  Seems like I should still be able to do what I want though.</p>