Refactor the refresh-wp-install-tests.sh script

The comment added now includes the source URL, and overall the script removes a fair number of backslashes.

This commit also contains code from @szepeviktor, see #5

bin/refresh-wp-install-tests.sh

@@ -2,10 +2,13 @@
 #
 # Refreshes the bin/install-wp-tests.sh file from source, injecting a comment at the top of the file.
 
-SRC="https://raw.githubusercontent.com/wp-cli/scaffold-command/master/templates/install-wp-tests.sh"
-DIR="$( cd "$(dirname "$0")" || exit 1; pwd -P )"
+upstream_src='https://raw.githubusercontent.com/wp-cli/scaffold-command/master/templates/install-wp-tests.sh'
+bin_dir="$(dirname "${BASH_SOURCE[0]}")"
 
-curl -s "$SRC" \
-    | sed $'2i\\\n# Notice: This file is pulled from the WP-CLI scaffold command,\\\n# and should not be modified directly.\n' \
-    > "$DIR/install-wp-tests.sh" \
-    && echo -e "\033[0;32mbin/install-wp-tests.sh has been updated to match ${SRC}\033[0;0m"
+curl -s "$upstream_src" | sed "2i\\
+# Notice: This file is pulled from the WP-CLI scaffold command, and should not be modified directly.\\
+#\\
+# ${upstream_src}
+" > "${bin_dir}/install-wp-tests.sh" || exit 1
+
+printf "\033[0;32m%s %s\033[0;32m\n" 'bin/install-wp-tests.sh has been updated to match' "$upstream_src"