Add [make undo].

Makefile

@@ -162,3 +162,9 @@ release:
 publish:
 # Publish an opam description.
 	@ opam publish -v $(DATE) $(THIS) $(ARCHIVE) .
+
+.PHONY: undo
+undo:
+# Undo the last release (assuming it was done on the same date).
+	@ git tag -d $(DATE)
+	@ git push -u origin :$(DATE)