Fix order for merging iterator

Consider the following YAML:

    trait1: &t1
      foo: 1

    trait2: &t2
      foo: 2

    merged:
      <<: *t1
      <<: *t2

yq reports:

    $ yq .merged.foo < /tmp/yaml
    2

while the order that yaml-cpp returns is different, since it will
firstly handle 1, and will not replace it with 2:

    $ util/parse < /tmp/yaml
    trait1:
      ? &1 foo
      : &2 1
    trait2:
      foo: 2
    merged:
      *1 : *2

(Don't mix up "*2" with "2", it is trait1)

src/nodebuilder.cpp

@@ -97,8 +97,9 @@ void NodeBuilder::OnMapEnd() {
   assert(m_mapDepth > 0);
   detail::node& collection = *m_stack.back();
   auto& toMerge = *m_mergeDicts.rbegin();
-  for (detail::node* n : toMerge) {
-    MergeMapCollection(collection, *n, m_pMemory);
+  /// The elements for merging should be traversed in reverse order to prefer last values.
+  for (auto it = toMerge.rbegin(); it != toMerge.rend(); ++it) {
+    MergeMapCollection(collection, **it, m_pMemory);
   }
   m_mapDepth--;
   m_mergeDicts.pop_back();

test/integration/load_node_test.cpp

@@ -199,6 +199,23 @@ TEST(LoadNodeTest, MergeKeyAIterator) {
   ASSERT_EQ(z_b_keys, 1);
 }
 
+TEST(LoadNodeTest, MergeKeyTwoOverrides) {
+  Node node = Load(R"(
+trait1: &t1
+  foo: 1
+
+trait2: &t2
+  foo: 2
+
+merged:
+  <<: *t1
+  <<: *t2
+)");
+  EXPECT_EQ(NodeType::Map, node["merged"].Type());
+  EXPECT_FALSE(node["merged"]["<<"]);
+  EXPECT_EQ(2, node["merged"]["foo"].as<int>());
+}
+
 TEST(LoadNodeTest, MergeKeyB) {
   Node node = Load(
       "{x: &foo {a : 1,b : 1,c : 1}, y: &bar {d: 2, e : 2, f : 2, a : 2}, z: "
@@ -211,7 +228,7 @@ TEST(LoadNodeTest, MergeKeyB) {
   EXPECT_EQ(3, node["z"]["b"].as<int>());
   EXPECT_EQ(1, node["z"]["c"].as<int>());
 
-  EXPECT_EQ(1, node["w"]["a"].as<int>());
+  EXPECT_EQ(2, node["w"]["a"].as<int>());
   EXPECT_EQ(3, node["w"]["b"].as<int>());
   EXPECT_EQ(4, node["w"]["c"].as<int>());
   EXPECT_EQ(2, node["w"]["d"].as<int>());