title: ng-repeat call controller function too many times
body: <p>I have next code (events is array):</p>
  
  <pre><code>&lt;tr ng-repeat="event in events"&gt;
      &lt;td&gt;
          &lt;span time-ago="{{event.TimestampSec}}"&gt;&lt;/span&gt;
      &lt;/td&gt;
      &lt;td&gt;
          {{prepareAlertValue(event.AlertValue)}}
      &lt;/td&gt;
  &lt;/tr&gt;
  </code></pre>
  
  <p>time-ago - my custom directive. It is executed <em>events.length</em> times.</p>
  
  <p>My controller:</p>
  
  <pre><code>...
  window.callPrepareAlertValueCount = 0
  
  $scope.prepareAlertValue = function(value) {
      window.callPrepareAlertValueCount++;
      if(safemineHelper.isFloat(value)) {
          value = (~~(value * 100)) / 100;
      }
      return value;
  }
  ...
  </code></pre>
  
  <p>After list is shown - I see that <em>callPrepareAlertValueCount</em> grows. Console log:</p>
  
  <pre><code> &gt; callPrepareAlertValueCount
   &lt; 41890
   &gt; callPrepareAlertValueCount
   &lt; 46150
   &gt; callPrepareAlertValueCount
   &lt; 480315
  </code></pre>
  
  <p>Please, can someone explain why is prepareAlertValue executed all time. 
  Do I need to write directives for each formatter function?</p>