#679: keep the bonus earned up to the threshold

[morphqdd]

At hoc == hoc_threshold the whole hits-of-code bonus was thrown away and the
penalty was added at the same instant, so one extra line in the diff cost 16
points with the shipped defaults.

The bonus is now counted up to the threshold and kept there, and only the penalty
applies above it, so the transition is the penalty itself and nothing more.

The expected numbers in test_check_all_bills move with it, for the four inputs
at or above the threshold.

Closes #679

[yegor256]

Thanks — smooths the hits-of-code cliff at the threshold, with a regression test and updated bill expectations. CI is green.

@rultor merge

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