rust: less set lookups in AncestorsIterator This uses the boolean return of `HashSet::insert()` to factor pairs of contains()/insert() into a single insert() On the mozilla-central repository (450k changesets), I get about a bit more than 10% better medians in perfancestors (taking the mean of three runs) Best run for parent changeset: ! wall 0.106474 comb 0.110000 user 0.110000 sys 0.000000 (median of 93) Best run for this changeset: ! wall 0.093191 comb 0.090000 user 0.090000 sys 0.000000 (median of 100) Differential Revision: https://phab.mercurial-scm.org/D5942