rust: use .rev() for reverse range

rust: use .rev() for reverse range

Differential Revision: https://phab.mercurial-scm.org/D5450

--- a/rust/hg-core/src/ancestors.rs	Fri Jan 04 00:00:44 2019 +0530
+++ b/rust/hg-core/src/ancestors.rs	Tue Dec 18 22:12:16 2018 +0900
@@ -321,7 +321,7 @@
 
         // TODO heuristics for with_capacity()?
         let mut missing: Vec<Revision> = Vec::new();
-        for curr in (0..=start).map(|i| start - i) {
+        for curr in (0..=start).rev() {
             if revs_visit.is_empty() {
                 break;
             }