Picking this back up after a little while. This solution means we can still accept -L, for an empty file but any other range will fail. I've made the change for both blame and log (as two separate patches).
I've also changed behaviour in a couple of corner cases - before we couldn't distinguish between -Ln,-(n+1) and -Ln, so -Ln,-(n+1) would blame from n to the end of the file rather than the first n lines. Also, we now complain that -L,-n is an empty range where previously this would blame the whole file.
