Added "show_upgrade_diffs" script (#981)

* Added "show_upgrade_diffs" script

"show_upgrade_diffs" pulls a git directory and a user branch from nomulus and
compares all of the versions of all dependencies specified in all lockfiles in
the master branch with those of the user branch and prints a nice, terse
little colorized report on the differences.

This is useful for reviewing a dependency upgrade.

* Add license header

* Changes requested in review

* Changes for review

- Change format of output so different actions are displayed somewhat
  consistently.
- Make specifying a directory optional, if not specified create a temporary
  directory and clean it up afterwards.

show_upgrade_diffs

@@ -0,0 +1,18 @@
+#!/bin/sh
+# Copyright 2021 The Nomulus Authors. All Rights Reserved.
+#
+# 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
+#
+#     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.
+#
+# Wrapper for show_upgrade_diffs.py.
+python3 "$(dirname $0)/config/show_upgrade_diffs.py" "$@"
+exit $?