Touch up checkformat.bash

The logic can be slightly simplified,
and the script doesn't require Bash after all.

contrib/checkformat.sh

@@ -0,0 +1,15 @@
+#!/usr/bin/env sh
+
+# SPDX-License-Identifier: MIT
+
+clang-format --version
+
+find . -type f -iname '*.[ch]pp' -exec clang-format -i {} +
+
+if ! git diff-index --quiet HEAD --; then
+	echo 'Unformatted files:'
+	git diff-index --name-only HEAD --
+	echo
+	git diff HEAD --
+	exit 1
+fi