[msgpack] explicitly give # of items of NHITS
[msgpack] explicitly give # of items of NHITS

NHITS always has exactly one element.

groonga/groonga,groonga/groonga,redfigure/groonga,hiroyuki-sato/groonga,kenhys/groonga,cosmo0920/groonga,hiroyuki-sato/groonga,naoa/groonga,hiroyuki-sato/groonga,naoa/groonga,cosmo0920/groonga,komainu8/groonga,groonga/groonga,naoa/groonga,cosmo0920/groonga,kenhys/groonga,naoa/groonga,cosmo0920/groonga,redfigure/groonga,kenhys/groonga,naoa/groonga,myokoym/groonga,naoa/groonga,kenhys/groonga,groonga/groonga,myokoym/groonga,komainu8/groonga,groonga/groonga,kenhys/groonga,cosmo0920/groonga,komainu8/groonga,cosmo0920/groonga,redfigure/groonga,komainu8/groonga,redfigure/groonga,cosmo0920/groonga,cosmo0920/groonga,komainu8/groonga,myokoym/groonga,hiroyuki-sato/groonga,redfigure/groonga,naoa/groonga,myokoym/groonga,hiroyuki-sato/groonga,redfigure/groonga,redfigure/groonga,groonga/groonga,myokoym/groonga,hiroyuki-sato/groonga,komainu8/groonga,groonga/groonga,naoa/groonga,myokoym/groonga,groonga/groonga,komainu8/groonga,redfigure/groonga,myokoym/groonga,komainu8/groonga,kenhys/groonga,hiroyuki-sato/groonga,hiroyuki-sato/groonga,kenhys/groonga,kenhys/groonga,myokoym/groonga