code: @Override
      public Void call() throws TTException {
          updateOnly();
          if (mCommit == EShredderCommit.COMMIT) {
              mWtx.commit();
          }
          return null;
      }
docs: Invoking the shredder.
  
  @throws TTException
  if Treetank encounters something went wrong
  @return revision of last revision (before commit)