Add scripts for interacting with GitHub (#2476)

These scripts automate some common maintainer tasks. They use the
[gh cli](https://github.com/cli/cli).

scripts/git/main_to_google.sh

diff --git a/scripts/git/main_to_google.sh b/scripts/git/main_to_google.sh
new file mode 100755
index 0000000..4de198a
--- /dev/null
+++ b/scripts/git/main_to_google.sh
@@ -0,0 +1,71 @@
+#!/bin/bash
+
+# Copyright 2020 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Checks out a branch at the specified green commit (default "main") and
+# creates a PR from that branch based on google.
+#
+# - Requries the gh CLI (https://github.com/cli/cli) to create a PR.
+# - Will force push to the configured PR_BRANCH (default "main-to-google") on
+#   the configured FORK_REMOTE (default "fork")
+# - Requires that local "main" branch is a pristine (potentially stale) copy
+#   of the "main" branch on the configured UPSTREAM_REMOTE
+#   (default "upstream").
+# - Requires that the working directory be clean. Will abort otherwise.
+
+set -x
+set -e
+set -o pipefail
+
+GREEN_COMMIT="${1:-main}"
+
+PR_BRANCH="${PR_BRANCH:-main-to-google}"
+UPSTREAM_REMOTE="${UPSTREAM_REMOTE:-upstream}"
+FORK_REMOTE="${FORK_REMOTE:-fork}"
+
+if [[ -n "$(git status --porcelain)" ]]; then
+  echo "Working directory not clean. Aborting"
+  git status
+  exit 1
+fi
+if ! git symbolic-ref -q HEAD; then
+  echo "In a detached HEAD state. Aborting"
+  git status
+  exit 1
+fi
+git checkout main
+git pull "${UPSTREAM_REMOTE?}" main --ff-only
+git submodule update --init
+if [[ -n "$(git status --porcelain)" ]]; then
+  echo "Working directory not clean after sync. Aborting"
+  git status
+  exit 1
+fi
+if [[ "${GREEN_COMMIT}" != "main" ]]; then
+  git checkout "${GREEN_COMMIT?}"
+  git submodule update
+fi
+git checkout -B "${PR_BRANCH?}"
+git push -f "${FORK_REMOTE?}" "${PR_BRANCH?}"
+if [[ -z "$(which gh)" ]]; then
+  echo "gh not found on path."
+  echo "Have you installed the GitHub CLI (https://github.com/cli/cli)?"
+  echo "Cannot create PR. Branch ${BRANCH?} pushed, but aborting."
+  exit 1
+fi
+gh pr create \
+    --base google \
+    --title="Merge main -> google" \
+    --body="$(git log google.. --decorate=no --pretty='format:* %h %<(80,trunc)%s')"