Fix shutdown for thin client shell

When a user enters shutdown in the thin client console, it only exits
the thin client, it does not actually shutdown sbt. Running `sbtn
shutdown` did work to shutdown the server, however. It turned out that
this was because there was special handling for shutdown when processed
through jline. We would enqueue the shutdown command and also close the
client connection. Closing the client connection though removed all of
the enqueued commands for the client, which included the shutdown
command. To fix this, we just make sure that we don't remove the
shutdown command when clearing the client commands.

main/src/main/scala/sbt/internal/CommandExchange.scala

@@ -160,7 +160,9 @@ private[sbt] final class CommandExchange {
     channelBufferLock.synchronized {
       Util.ignoreResult(channelBuffer -= c)
     }
-    commandQueue.removeIf(_.source.map(_.channelName) == Some(c.name))
+    commandQueue.removeIf { e =>
+      e.source.map(_.channelName) == Some(c.name) && e.commandLine != Shutdown
+    }
     currentExec.filter(_.source.map(_.channelName) == Some(c.name)).foreach { e =>
       Util.ignoreResult(NetworkChannel.cancel(e.execId, e.execId.getOrElse("0")))
     }