vendor: Update github.com/gogo/protobuf

Also tweaks the proto definitions:

 - [packed=false] on the block_indexes field to retain compat with
   v0.14.16 and earlier.

 - Uses the vendored protobuf package in include paths.

And, "build.go setup" will install the vendored protoc-gen-gogofast.
This should ensure that a proto rebuild isn't so dependent on whatever
version of the compiler and package the developer has installed...

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3864

vendor/github.com/gogo/protobuf/proto/clone.go

@@ -84,14 +84,20 @@ func mergeStruct(out, in reflect.Value) {
 		mergeAny(out.Field(i), in.Field(i), false, sprop.Prop[i])
 	}
 
-	if emIn, ok := in.Addr().Interface().(extensionsMap); ok {
-		emOut := out.Addr().Interface().(extensionsMap)
-		mergeExtension(emOut.ExtensionMap(), emIn.ExtensionMap())
-	} else if emIn, ok := in.Addr().Interface().(extensionsBytes); ok {
+	if emIn, ok := in.Addr().Interface().(extensionsBytes); ok {
 		emOut := out.Addr().Interface().(extensionsBytes)
 		bIn := emIn.GetExtensions()
 		bOut := emOut.GetExtensions()
 		*bOut = append(*bOut, *bIn...)
+	} else if emIn, ok := extendable(in.Addr().Interface()); ok {
+		emOut, _ := extendable(out.Addr().Interface())
+		mIn, muIn := emIn.extensionsRead()
+		if mIn != nil {
+			mOut := emOut.extensionsWrite()
+			muIn.Lock()
+			mergeExtension(mOut, mIn)
+			muIn.Unlock()
+		}
 	}
 
 	uf := in.FieldByName("XXX_unrecognized")