code: function _checkNewHash(filename, data) {
      var shasum = crypto.createHash('sha1');
      shasum.update(data);
      var hash = shasum.digest('hex');
      if (hash === hashes[filename]) {
        return null;
      }
      return hash;
    }
docs: /*
  Private functions.
   Check if file is differrent. Return the new hash if it is.