Add a git fetch before git reset --hard origin/master in big_red_button.py

sky/tools/big_red_button.py

 #!/usr/bin/env python
 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Prepare release script.
 #
 # 1) Bump versions:
 # sky/sdk/example/demo_launcher/apk/AndroidManifest.xml
 # mojo/dart/mojo_services/pubspec.yaml
@@ skipping 72 matching lines @@
     dart_sdk_root = os.path.join(mojo_root, 'third_party/dart-sdk/dart-sdk')
     pub_path = os.path.join(dart_sdk_root, 'bin/pub')
     packages_root = os.path.join(sky_sdk_root, 'packages')
 
     run(mojo_root, ['git', 'pull', '--rebase'])
     run(mojo_root, ['gclient', 'sync'])
     run(mojo_root, ['mojo/tools/mojob.py', 'gn', '--android', '--release'])
     run(mojo_root, ['mojo/tools/mojob.py', 'build', '--android', '--release'])
     # Run tests?
 
+    run(sky_sdk_root, ['git', 'fetch'])
     run(sky_sdk_root, ['git', 'reset', '--hard', 'origin/master'])
     run(mojo_root, [
         'sky/tools/deploy_sdk.py',
         '--non-interactive',
         '--commit',
         sky_sdk_root
     ])
     # tag for version?
 
+    run(demo_site_root, ['git', 'fetch'])
     run(demo_site_root, ['git', 'reset', '--hard', 'origin/master'])
     run(mojo_root, ['mojo/tools/deploy_domokit_site.py', demo_site_root])
     # tag for version?
 
     for package in os.listdir(packages_root):
         package_path = os.path.join(packages_root, package)
         if not os.path.isdir(package_path):
             continue
         run(package_path, [pub_path, 'publish', '--force'])
 
     run(sky_sdk_root, ['git', 'push'])
     run(demo_site_root, ['git', 'push'])
 
 
 if __name__ == '__main__':
     sys.exit(main())