mk: Implement git cleanliness check

Makefile

@@ -8,6 +8,13 @@ sign: $(patsubst %,%.sig,$(wildcard *.zone))
 %.sig: %
 	gpg -o $@ --detach-sign $<
 
+.PHONY: check-clean
+check-clean:
+	[ ! -e .git ] || git diff-index --exit-code HEAD
+	[ ! -e .git ] || \
+		git ls-files --other --directory --exclude-standard | \
+		sed -n '/.*\.zone/{p;q1}'
+
 upload:
 	printf 'put %s\n' *.zone *.sig | sftp dns.parabox.it-syndikat.org:zones/