Upgrade go-github to v39 (#17437)

vendor/github.com/google/go-github/v39/github/with_appengine.go

@@ -0,0 +1,21 @@
+// Copyright 2017 The go-github AUTHORS. All rights reserved.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build appengine
+// +build appengine
+
+// This file provides glue for making github work on App Engine.
+
+package github
+
+import (
+	"context"
+	"net/http"
+)
+
+func withContext(ctx context.Context, req *http.Request) *http.Request {
+	// No-op because App Engine adds context to a request differently.
+	return req
+}