Improve analysis of the number of extra `==` operations, which was overly complicated, slightly incorrect.
The old analysis viewed it as birthday attack, which asks how often
there are multiple values in the probe same probe sequence with the
same H2. In their own words, this analysis "breaks down" at around `n
= 12`.
Instead we can answer a simpler question, which is, if a probe
sequence examines `k` objects that are not what we are looking for,
what's the number of calls `==`? The expectation is simply `k/128`.
PiperOrigin-RevId: 446518063
Change-Id: Ie879bd4f6c97979822bc9d550b9e2503b1418c78
