index: implement AsCompositeIndex for CompositeIndex, not for &Compos…

…iteIndex

Just a minor code cleanup. We still need Index for &CompositeIndex because the
type is unsized, and unsized type cannot be converted to another dyn reference.

lib/src/default_index/composite.rs

@@ -399,12 +399,13 @@ impl CompositeIndex {
     }
 }
 
-impl AsCompositeIndex for &CompositeIndex {
+impl AsCompositeIndex for CompositeIndex {
     fn as_composite(&self) -> &CompositeIndex {
         self
     }
 }
 
+// In revset engine, we need to convert &CompositeIndex to &dyn Index.
 impl Index for &CompositeIndex {
     /// Suppose the given `commit_id` exists, returns the minimum prefix length
     /// to disambiguate it. The length to be returned is a number of hexadecimal