Use MacOSXWatchService instead of PollingWatchService

This watch service should be more responsive and significantly reduce
the disk overhead of the polling based service for large repos.

main-command/src/main/scala/sbt/Watched.scala

@@ -141,7 +141,7 @@ object Watched {
         FileSystems.getDefault.newWatchService()
       case _ if Properties.isMac =>
         // WatchService is slow on macOS - use old polling mode
-        new PollingWatchService(PollDelay)
+        new MacOSXWatchService(PollDelay)
       case _ =>
         FileSystems.getDefault.newWatchService()
     }