title: Failed to execute "arc diff": not found configuration
body: <p>Here is the logging of the execution of <code>arc diff</code>:</p>
  
  <pre><code>$ arc diff --trace
   ARGV  '/Users/yangyan/Meican/arcanist/bin/../scripts/arcanist.php' 'diff' '--trace'
   LOAD  Loaded "phutil" from "/Users/yangyan/Meican/libphutil/src".
   LOAD  Loaded "arcanist" from "/Users/yangyan/Meican/arcanist/src".
  Config: Reading user configuration file "/Users/yangyan/.arcrc"...
  Config: Did not find system configuration at "/etc/arcconfig".
  Working Copy: Reading .arcconfig from "/Users/yangyan/Meican/go/src/code.meican.com/diffusion/DEMETER/demeter.git/.arcconfig".
  Working Copy: Path "/Users/yangyan/Meican/go/src/code.meican.com/diffusion/DEMETER/demeter.git" is part of `git` working copy "/Users/yangyan/Meican/go/src/code.meican.com/diffusion/DEMETER/demeter.git".
  Working Copy: Project root is at "/Users/yangyan/Meican/go/src/code.meican.com/diffusion/DEMETER/demeter.git".
  Config: Did not find local configuration at "/Users/yangyan/Meican/go/src/code.meican.com/diffusion/DEMETER/demeter.git/.git/arc/config".
  &gt;&gt;&gt; [0] &lt;conduit&gt; user.whoami() &lt;bytes = 117&gt;
  &gt;&gt;&gt; [1] &lt;http&gt; https://code.meican.com/api/user.whoami
  &lt;&lt;&lt; [1] &lt;http&gt; 636,235 us
  &lt;&lt;&lt; [0] &lt;conduit&gt; 636,726 us
  
  [2016-04-01 06:33:53] EXCEPTION: (ConduitClientException) ERR-INVALID-SESSION: Session key is not present. at [&lt;phutil&gt;/src/conduit/ConduitFuture.php:58]
  arcanist(head=master, ref.master=fcc11b3a2781), phutil(head=master, ref.master=3024f0a4908b)
    #0 ConduitFuture::didReceiveResult(array) called at [&lt;phutil&gt;/src/future/FutureProxy.php:58]
    #1 FutureProxy::getResult() called at [&lt;phutil&gt;/src/future/FutureProxy.php:35]
    #2 FutureProxy::resolve() called at [&lt;phutil&gt;/src/conduit/ConduitClient.php:58]
    #3 ConduitClient::callMethodSynchronous(string, array) called at [&lt;arcanist&gt;/src/workflow/ArcanistWorkflow.php:332]
    #4 ArcanistWorkflow::authenticateConduit() called at [&lt;arcanist&gt;/scripts/arcanist.php:354]
  </code></pre>
  
  <p>How should I treat the following errors?</p>
  
  <p>Config: Did not find system configuration at "/etc/arcconfig".
  Config: Did not find local configuration at "/Users/yangyan/Meican/go/src/code.meican.com/diffusion/DEMETER/demeter.git/.git/arc/config".</p>