proto: import change from Google.

Add tests for merging a map field. When src contains a duplicate key,
its value message replaces (not merges with) the value message in dst.

proto/clone_test.go

diff --git a/proto/clone_test.go b/proto/clone_test.go
index 76720f1..f607ff4 100644
--- a/proto/clone_test.go
+++ b/proto/clone_test.go
@@ -195,6 +195,9 @@
 			NameMapping: map[int32]string{6: "Nigel"},
 			MsgMapping: map[int64]*pb.FloatingPoint{
 				0x4001: &pb.FloatingPoint{F: proto.Float64(2.0)},
+				0x4002: &pb.FloatingPoint{
+					F: proto.Float64(2.0),
+				},
 			},
 			ByteMapping: map[bool][]byte{true: []byte("wowsa")},
 		},
@@ -203,6 +206,12 @@
 				6: "Bruce", // should be overwritten
 				7: "Andrew",
 			},
+			MsgMapping: map[int64]*pb.FloatingPoint{
+				0x4002: &pb.FloatingPoint{
+					F:     proto.Float64(3.0),
+					Exact: proto.Bool(true),
+				}, // the entire message should be overwritten
+			},
 		},
 		want: &pb.MessageWithMap{
 			NameMapping: map[int32]string{
@@ -211,6 +220,9 @@
 			},
 			MsgMapping: map[int64]*pb.FloatingPoint{
 				0x4001: &pb.FloatingPoint{F: proto.Float64(2.0)},
+				0x4002: &pb.FloatingPoint{
+					F: proto.Float64(2.0),
+				},
 			},
 			ByteMapping: map[bool][]byte{true: []byte("wowsa")},
 		},
@@ -254,6 +266,27 @@
 			Union: &pb.Communique_Name{"Bobby Tables"},
 		},
 	},
+	{
+		src: &proto3pb.Message{
+			Terrain: map[string]*proto3pb.Nested{
+				"kay_a": &proto3pb.Nested{Cute: true},      // replace
+				"kay_b": &proto3pb.Nested{Bunny: "rabbit"}, // insert
+			},
+		},
+		dst: &proto3pb.Message{
+			Terrain: map[string]*proto3pb.Nested{
+				"kay_a": &proto3pb.Nested{Bunny: "lost"},  // replaced
+				"kay_c": &proto3pb.Nested{Bunny: "bunny"}, // keep
+			},
+		},
+		want: &proto3pb.Message{
+			Terrain: map[string]*proto3pb.Nested{
+				"kay_a": &proto3pb.Nested{Cute: true},
+				"kay_b": &proto3pb.Nested{Bunny: "rabbit"},
+				"kay_c": &proto3pb.Nested{Bunny: "bunny"},
+			},
+		},
+	},
 }
 
 func TestMerge(t *testing.T) {

proto/proto3_proto/proto3.proto

diff --git a/proto/proto3_proto/proto3.proto b/proto/proto3_proto/proto3.proto
index 31509e4..3e9a1ab 100644
--- a/proto/proto3_proto/proto3.proto
+++ b/proto/proto3_proto/proto3.proto
@@ -66,6 +66,7 @@
 
 message Nested {
   string bunny = 1;
+  bool cute = 2;
 }
 
 message MessageWithMap {

proto/testdata/test.proto

diff --git a/proto/testdata/test.proto b/proto/testdata/test.proto
index f607113..2ce2960 100644
--- a/proto/testdata/test.proto
+++ b/proto/testdata/test.proto
@@ -488,6 +488,7 @@
 
 message FloatingPoint {
   required double f = 1;
+  optional bool exact = 2;
 }
 
 message MessageWithMap {