Send exit in network client before shutdown

In the client test, the sbt server would keep open the the client
connection even after it had exited because the client was only shutting
down its side of the connection. Since in the test it wasn't exiting the
jvm, the read side of the connection was still open.

main-command/src/main/scala/sbt/internal/client/NetworkClient.scala

@@ -919,7 +919,9 @@ class NetworkClient(
       if (mainThread != null && mainThread != Thread.currentThread) mainThread.interrupt
       connectionHolder.get match {
         case null =>
-        case c    => c.shutdown()
+        case c =>
+          try sendExecCommand("exit")
+          finally c.shutdown()
       }
       Option(inputThread.get).foreach(_.interrupt())
     } catch {