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

rust/hg-core/src/ancestors.rs

@@ -79,8 +79,7 b' impl<G: Graph> AncestorsIterator<G> {'
 
     #[inline]
     fn conditionally_push_rev(&mut self, rev: Revision) {
-        if self.stoprev <= rev && !self.seen.contains(&rev) {
-            self.seen.insert(rev);
+        if self.stoprev <= rev && self.seen.insert(rev) {
             self.visit.push(rev);
         }
     }
@@ -154,11 +153,10 b' impl<G: Graph> Iterator for AncestorsIte'
             Ok(ps) => ps,
             Err(e) => return Some(Err(e)),
         };
-        if p1 < self.stoprev || self.seen.contains(&p1) {
+        if p1 < self.stoprev || !self.seen.insert(p1) {
             self.visit.pop();
         } else {
             *(self.visit.peek_mut().unwrap()) = p1;
-            self.seen.insert(p1);
         };
 
         self.conditionally_push_rev(p2);