Sacramento record label Dig Music is best known for releasing records by hometown favorites Jackie Greene, Bucho and Kate Gaffney, as well as classic rock artists Sal Valentino and Cold Blood.     But right now, the label's best-selling artist, with an album at No.3 on the Jazzweek chart, is a release by another label, Unity, in a genre Dig Music has had very little experience with: Jazz.     Jazz trumpeter Scotty Barnhart is signed with Unity Records, but his solo debut, Say It Plain, was released and distributed in May by Dig Music.     Barnhart is doing business in Northern California, although he spends his time either at home in Los Angeles or Tallahassee where he teaches pa