title: git: can't push (unpacker error) related to permission issues
body: <p>I have this problem when i try to push in git:</p>
  
  <pre><code>error: insufficient permission for adding an object to repository database ./objects
  
  fatal: failed to write object
  error: unpack failed: unpack-objects abnormal exit
  To ssh://&lt;repo url&gt;/&lt;repo dir&gt;
   ! [remote rejected] master -&gt; master (n/a (unpacker error))
  error: failed to push some refs to 'ssh://&lt;repo url&gt;/&lt;repo dir&gt;'
  </code></pre>
  
  <p>I've had this before sporadically and we've always had to solve it by each user sshing to the repo and setting group permissions on all the files therein with </p>
  
  <pre><code>chmod -R g+w *
  </code></pre>
  
  <p>This was never a satisfactory solution and now it's bitten us in the arse as one of the guys is away and no-one knows his repo user's password.  So, i'm trying to solve it properly.  </p>
  
  <p>The error seems to occur when someone tries to push up a change that will alter a repo dir that is owned by another user (hence setting the group write option above).  I've done a bit of googling around this and have found a couple of solutions being discussed (neither of which worked for me)</p>
  
  <p>1) make sure that the group that the repo dirs are shared with is each users' primary group (i believe that is the case already: each user has only one group so that must be their primary group, right?)</p>
  
  <p>2) git repo core.sharedRepository setting, as detailed here: <a href="https://stackoverflow.com/questions/398203/git-cant-push-from-one-computer#answer-399133">Git: Can&#39;t push from one computer</a>
  I changed this but it didn't make any difference.  Do i need to reload the config or something to actually effect the change?</p>
  
  <p>Here's what my repo config looks like atm:</p>
  
  <pre><code>[core]
          repositoryformatversion = 0
          filemode = true
          bare = true
          sharedRepository = all
  [receive]
          denyNonFastForwards = True
  </code></pre>
  
  <p>Grateful for any advice or suggestions!
  max</p>