fix command sender flush bug

mirai-console/src/main/kotlin/net/mamoe/mirai/console/command/Command.kt

@@ -106,13 +106,13 @@ interface CommandSender {
 }
 
 abstract class CommandSenderImpl : CommandSender {
-    private val builder = StringBuilder()
+    internal val builder = StringBuilder()
 
     override fun appendMessage(message: String) {
         builder.append(message).append("\n")
     }
 
-    internal suspend fun flushMessage() {
+    internal open suspend fun flushMessage() {
         if (!builder.isEmpty()) {
             sendMessage(builder.toString().removeSuffix("\n"))
         }
@@ -127,6 +127,11 @@ object ConsoleCommandSender : CommandSenderImpl() {
     override suspend fun sendMessage(message: String) {
         MiraiConsole.logger("[Command]", 0, message)
     }
+
+    override suspend fun flushMessage() {
+        super.flushMessage()
+        builder.clear()
+    }
 }
 
 open class ContactCommandSender(val contact: Contact) : CommandSenderImpl() {

mirai-console/src/main/kotlin/net/mamoe/mirai/console/command/DefaultCommands.kt

@@ -237,6 +237,7 @@ object DefaultCommands {
             onCommand {
                 CommandManager.getCommands().let {
                     var size = 0
+                    appendMessage("")//\n
                     it.toSet().forEach {
                         ++size
                         appendMessage("-> " + it.name + " :" + it.description)