gh-115727: Reduce confidence even on 100% predicted jumps (#115748)

The theory is that even if we saw a jump go in the same direction the
last 16 times we got there, we shouldn't be overly confident that it's
still going to go the same way in the future. This PR makes it so that
in the extreme cases, the confidence is multiplied by 0.9 instead of
remaining unchanged. For unpredictable jumps, there is no difference
(still 0.5). For somewhat predictable jumps, we interpolate.