Shutdown progress thread when there are no tasks

It is still possible for progress threads to leak so shut them down if
there are no active tasks. The report0 method will start up a new thread
if a task is added.

main/src/main/scala/sbt/internal/TaskProgress.scala

@@ -35,6 +35,7 @@ private[sbt] final class TaskProgress(log: ManagedLogger)
         try {
           report()
           Thread.sleep(sleepDuration)
+          if (active.isEmpty) TaskProgress.this.stop()
         } catch {
           case _: InterruptedException =>
         }
@@ -58,7 +59,9 @@ private[sbt] final class TaskProgress(log: ManagedLogger)
 
   override def afterCompleted[A](task: Task[A], result: Result[A]): Unit = ()
 
-  override def stop(): Unit = currentProgressThread.getAndSet(None).foreach(_.close())
+  override def stop(): Unit = currentProgressThread.synchronized {
+    currentProgressThread.getAndSet(None).foreach(_.close())
+  }
 
   override def afterAllCompleted(results: RMap[Task, Result]): Unit = {
     // send an empty progress report to clear out the previous report
@@ -80,8 +83,9 @@ private[sbt] final class TaskProgress(log: ManagedLogger)
       case _ =>
     }
   }
+  private[this] def active: Vector[Task[_]] = activeTasks.toVector.filterNot(Def.isDummy)
   private[this] def report(): Unit = {
-    val currentTasks = activeTasks.toVector.filterNot(Def.isDummy)
+    val currentTasks = active
     val ltc = lastTaskCount.get
     val currentTasksCount = currentTasks.size
     def report0(tasks: Vector[Task[_]]): Unit = {