rust: itering less on MissingAncestors.bases for max() Instead of iterating on the whole `self.bases` each time to find its max, we keep the latter in a separate member attribute and keep it up to date in `add_bases()` On a perfdiscovery done on PyPy, with repos prepared with `contrib/discovery-helper.sh 50 100`, this gives a slight improvement (around 0.5% on wall time, but 10% on CPU) before: ! wall 0.172801 comb 0.180000 user 0.180000 sys 0.000000 (median of 541) after: ! wall 0.171798 comb 0.160000 user 0.160000 sys 0.000000 (median of 551) (perf command run time upped because of bigger variability during this test). Differential Revision: https://phab.mercurial-scm.org/D5945