add scalafixJGitCompletion key. reduce memory usage

[xuwei-k]

Maybe partially(?) fix scalacenter/scalafix#1965 if there are many sub projects

if there is example sbt project as follow

lazy val p1 = project
lazy val p2 = project
lazy val p3 = project
lazy val p4 = project.disablePlugins(ScalafixPlugin)

before

(p1 + p2 + p3 + implicit root project) * ( Compile/scalafix + Test/scalafix + scalafixAll )
= 4 * 3
= 12JGitCompletion instances

after

only one JGitCompletion instance

[bjaglin]

Thanks for looking into that @xuwei-k !