fix mutableWithKV to actually use the new value

minor opt to some mutable methods - when converting from immutable to a new mutable Tree, the counts should be initial values (making the tree mutable), not the current counts (making it immutable)

Author: mkeskells

library/src/scala/collection/immutable/RedBlackTree.scala

@@ -584,14 +584,14 @@ private[collection] object NewRedBlackTree {
       else if (mutable) {
         _key = newKey
         this
-      } else new Tree(newKey, _value.asInstanceOf[AnyRef], _left, _right, _count)
+      } else new Tree(newKey, _value.asInstanceOf[AnyRef], _left, _right, initialCount)
     }
     def mutableWithV[B1 >: B](newValue: B1): Tree[A, B1] = {
       if (newValue.asInstanceOf[AnyRef] eq _value.asInstanceOf[AnyRef]) this
       else if (mutable) {
         _value = newValue.asInstanceOf[AnyRef]
         this
-      } else new Tree(_key, newValue.asInstanceOf[AnyRef], _left, _right, _count)
+      } else new Tree(_key, newValue.asInstanceOf[AnyRef], _left, _right, initialCount)
     }
     //Note - in 2.13 remove his method
     //due to the handling of keys in 2.13 we never replace a key
@@ -600,9 +600,9 @@ private[collection] object NewRedBlackTree {
         (newValue.asInstanceOf[AnyRef] eq _value.asInstanceOf[AnyRef])) this
       else if (mutable) {
         _key = newKey
-        _value = value.asInstanceOf[AnyRef]
+        _value = newValue.asInstanceOf[AnyRef]
         this
-      } else new Tree(newKey, newValue.asInstanceOf[AnyRef], _left, _right, _count)
+      } else new Tree(newKey, newValue.asInstanceOf[AnyRef], _left, _right, initialCount)
     }
     def mutableWithLeft[B1 >: B](newLeft: Tree[A, B1]): Tree[A, B1] = {
       if (_left eq newLeft) this

library/src/scala/collection/immutable/TreeMap.scala

@@ -287,7 +287,7 @@ final class TreeMap[A, +B] private (tree: RB.Tree[A, B])(implicit val ordering:
     private var currentMutableTree: RB.Tree[A,B1] = tree0
     def finalTree = beforePublish(currentMutableTree)
     override def apply(kv: (A, B1)): Unit = {
-      currentMutableTree= mutableUpd(currentMutableTree, kv._1, kv._2)
+      currentMutableTree = mutableUpd(currentMutableTree, kv._1, kv._2)
     }
     @tailrec def addAll(ls: LinearSeq[(A, B1)]): Unit = {
       if (!ls.isEmpty) {