[MINOR] Adding scripts to checkout and push to PRs (#2109)

- Tested the checkout_pr.sh locally
 - Tested a dryrun of pr_push_command.sh

scripts/checkout_pr.sh

@@ -0,0 +1,61 @@
+#!/bin/bash
+
+#  Licensed to the Apache Software Foundation (ASF) under one
+#  or more contributor license agreements.  See the NOTICE file
+#  distributed with this work for additional information
+#  regarding copyright ownership.  The ASF licenses this file
+#  to you 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
+#
+#      http://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.
+
+#
+# Usage: ./scripts/checkout_pr.sh
+#
+# Checkout a PR given the PR number into a local branch. PR branches are named
+# using the convention "pull/<PR_NUMBER>", to enable pr_push_command.sh to work
+# in tandem.
+#
+set -eou pipefail
+
+function printUsage {
+  echo "Usage: $(basename "${0}") [-r REMOTE] [-f] <PR_NUMBER>" 2>&1
+  echo '   -r  REMOTE remote to grab PR from (default: apache)'
+  echo '   -f         force overwrite of local branch (default: fail if exists)'
+  exit 1
+}
+
+if [[ ${#} -eq 0 ]]; then
+  printUsage
+fi
+
+REMOTE="origin"
+FORCE=""
+while getopts ":r:f" arg; do
+  case "${arg}" in
+    r)
+      REMOTE="${OPTARG}"
+      ;;
+    f)
+      FORCE="--force"
+      ;;
+    ?)
+      printUsage
+      ;;
+  esac
+done
+shift "$(($OPTIND -1))"
+
+# Debug output
+PR_NUM=$1
+
+# Checkout the PR into a local branch.
+git fetch ${REMOTE} pull/${PR_NUM}/head:pull/${PR_NUM} ${FORCE}
+git checkout pull/${PR_NUM}