POSIX disambiguation: use the same comparison algorithm for orbit and non-orbit tags.

POSIX disambiguation: use the same comparison algorithm for orbit and non-orbit tags.

Previously we needed a different algorithm for non-orbit tags, because
disambiguation was based on both start and end tags. Non-orbit start tags
cannot be compared incrementally, like orbit tags, becuse default value
may be discovered on a later step than non-default value. Non-orbit end
tags do not have this problem: since negative tags are inserted at the end
of alternatives, default value is always discovered on the same step as
non-default value (provided that all higher-priority tags agree and
comparison reaches this tag at all).

Now that start tags are ignored, we can use incremental comparison for both
orbit and non-orbit subhistories, which simplifies the code.