title: Gerrit - Internal server error while pushing to gerrit for review
body: <p>I've gerrit-2.11 installed in my server and I was testing email notification by pushing some changes to gerrit for review. After a number of pushes it started throwing following error.</p>
  
  <pre><code>$ git push origin HEAD:refs/for/master
  Counting objects: 4, done.Writing objects: 100% (3/3), 289 bytes, done.
  Total 3 (delta 0), reused 0 (delta 0)
  remote: Processing changes: refs: 2, done
  To ssh://user@host:29418/project
   ! [remote rejected] HEAD -&gt; refs/for/master (internal server error)
  error: failed to push some refs to 'ssh://user@host:29418/project'
  </code></pre>
  
  <p>I've a number of projects hosted on this server. Now I'm getting the above error for all projects while trying to push something for review. I can see following exceptions in the gerrit error log.</p>
  
  <pre><code>[2015-05-26 08:25:07,797] ERROR com.google.gerrit.server.git.ReceiveCommits : Can't insert change/patchset for project
  com.google.gerrit.server.git.InsertException: Error inserting change/patchset
          at com.google.gerrit.server.git.ReceiveCommits$1.apply(ReceiveCommits.java:269)
          at com.google.gerrit.server.git.ReceiveCommits$1.apply(ReceiveCommits.java:260)
          at com.google.common.util.concurrent.Futures$MappingCheckedFuture.mapException(Futures.java:1809)
          at com.google.common.util.concurrent.AbstractCheckedFuture.checkedGet(AbstractCheckedFuture.java:85)
          at com.google.gerrit.server.git.ReceiveCommits.insertChangesAndPatchSets(ReceiveCommits.java:803)
          at com.google.gerrit.server.git.ReceiveCommits.processCommands(ReceiveCommits.java:588)
          at com.google.gerrit.server.git.AsyncReceiveCommits$Worker.run(AsyncReceiveCommits.java:89)
          at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
          at com.google.gerrit.server.util.RequestScopePropagator$5.call(RequestScopePropagator.java:222)
          at com.google.gerrit.server.util.RequestScopePropagator$4.call(RequestScopePropagator.java:201)
          at com.google.gerrit.server.util.ThreadLocalRequestScopePropagator$1.call(ThreadLocalRequestScopePropagator.java:55)
  </code></pre>
  
  <p>*</p>