zstd: add script to import zstd from upstream

Utilities/Scripts/update-zstd.bash

+#!/usr/bin/env bash
+
+set -e
+set -x
+shopt -s dotglob
+
+readonly name="zstd"
+readonly ownership="zstd upstream <kwrobot@kitware.com>"
+readonly subtree="Utilities/cmzstd"
+readonly repo="https://github.com/facebook/zstd.git"
+readonly tag="v1.3.8"
+readonly shortlog=false
+readonly paths="
+  LICENSE
+  README.md
+  lib/common/*.c
+  lib/common/*.h
+  lib/compress/*.c
+  lib/compress/*.h
+  lib/decompress/*.c
+  lib/decompress/*.h
+  lib/deprecated/*.c
+  lib/deprecated/*.h
+  lib/dictBuilder/*.c
+  lib/dictBuilder/*.h
+  lib/zstd.h
+"
+
+extract_source () {
+    git_archive
+    pushd "${extractdir}/${name}-reduced"
+    echo "* -whitespace" > .gitattributes
+    popd
+}
+
+. "${BASH_SOURCE%/*}/update-third-party.bash"