PYTHON-981 - Optimize looking for $out in aggregation pipeline.

Author: Luke Lovett

pymongo/collection.py

@@ -1645,10 +1645,8 @@ def aggregate(self, pipeline, **kwargs):
  # Apply this Collection's read concern if $out is not in the
  # pipeline.
  if sock_info.max_wire_version >= 4 and 'readConcern' not in cmd:
- for stage in cmd['pipeline']:
- if '$out' in stage:
- result = self._command(sock_info, cmd, slave_ok)
- break
+ if pipeline and '$out' in pipeline[-1]:
+ result = self._command(sock_info, cmd, slave_ok)
  else:
  result = self._command(sock_info, cmd, slave_ok,
  read_concern=self.read_concern)