Fix buildNode() (and node_load()) called unnecessarily

In FeedsNodeProcessor.inc, the beginning of the process loop looks like this:

while ($item = $batch->shiftItem()) {

      // Create/update if item does not exist or update existing is enabled.
      if (!($nid = $this->existingItemId($item, $source)) || ($this->config['update_existing'] != FEEDS_SKIP_EXISTING)) {
        $node = $this->buildNode($nid, $source->feed_nid);

        // Only proceed if item has actually changed.
        $hash = $this->hash($item);
        if (!empty($nid) && $hash == $this->getHash($nid)) {
          continue;
        }
        $node->feeds_node_item->hash = $hash;

As far as I can tell there is no reason buildNode() can not go under the hash-check, avoiding a call to buildNode/node_load everytime Feeds comes across already existing data. Patch attached, all tests passing (including my application-specific tests which depend on FeedsNodeProcessor->process)