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/bazel_to_cmake.sh

diff --git a/scripts/git/bazel_to_cmake.sh b/scripts/git/bazel_to_cmake.sh
new file mode 100755
index 0000000..5b8234f
--- /dev/null
+++ b/scripts/git/bazel_to_cmake.sh
@@ -0,0 +1,66 @@
+#!/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.
+
+# Creates a PR based on the specified BASE_BRANCH (default "google") to fix
+# CMake files based on Bazel BUILD files.
+#
+# - Requries the gh CLI (https://github.com/cli/cli) to create a PR.
+# - Will force push to the configured PR_BRANCH (default "bazel-to-cmake-fix")
+#   on the configured FORK_REMOTE (default "fork")
+# - Requires that local BASE_BRANCH branch is a pristine (potentially stale)
+#   copy of the same branch on the configured UPSTREAM_REMOTE
+#   (default "upstream").
+# - Requires that the working directory be clean. Will abort otherwise.
+
+set -e
+set -x
+set -o pipefail
+
+BASE_BRANCH="${1:-google}"
+PR_BRANCH="bazel-to-cmake-fix"
+UPSTREAM_REMOTE="${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 "${BASE_BRANCH?}"
+git pull "${UPSTREAM_REMOTE?}" "${BASE_BRANCH?}" --ff-only
+git submodule update --init
+if [[ -n "$(git status --porcelain)" ]]; then
+  echo "Working directory not clean after sync. Aborting"
+  git status
+  exit 1
+fi
+git checkout -B "${PR_BRANCH?}"
+./build_tools/bazel_to_cmake/bazel_to_cmake.py
+
+TITLE="Run Bazel -> CMake"
+BODY='Updates CMake files to match Bazel BUILD
+
+\`./build_tools/bazel_to_cmake/bazel_to_cmake.py\`
+'
+
+git commit -am "${TITLE?}"
+git push -f "${FORK_REMOTE}" "${PR_BRANCH?}"
+gh pr create --title="${TITLE?}" --body="${BODY?}" --base="${BASE_BRANCH?}"