Fix non-determinism after sync; use issue id to resolve ties

[karlicoss]

Ok I think I figured that out... https://github.com/arbox/org-sync/issues/55

As a byproduct it's possible now to specify arbitrary key (e.g. you might want to sort by creation date only). But frankly I don't really like code I got as a result, so happy if you suggest a way to improve it..