rust: less set lookups in MissingAncestors using the return values of HashSet::remove(), we can factor pairs of `contains()/remove()` into a single `remove()`. On a perfdiscovery run done on the PyPy repository, prepared with contrib/discovery-helper.sh 50 100, I do get a modest improvement with this (mean of medians of three runs is better by 2%) Sample readings, before this change: ! wall 0.175609 comb 0.180000 user 0.180000 sys 0.000000 (median of 58) With this change: ! wall 0.171662 comb 0.180000 user 0.170000 sys 0.010000 (median of 60) Differential Revision: https://phab.mercurial-scm.org/D5943