revset: detach index from predicate fn, turn it into position-based

This is the step towards removing &CompositeIndex references from the revset
evaluation tree. The filter input is changed from &IndexEntry to IndexPosition
to simplify the lifetime thingy. We might want to pass around CommitId or
Commit object once it's loaded, but that can be implemented later. I don't
see significant performance difference in revset benches.

lib/src/default_index/rev_walk.rs

@@ -80,7 +80,6 @@ pub(super) struct EagerRevWalk<T> {
 }
 
 impl<T: Iterator> EagerRevWalk<T> {
-    #[cfg(test)] // TODO
     pub fn new(iter: T) -> Self {
         EagerRevWalk { iter: iter.fuse() }
     }
@@ -136,7 +135,6 @@ impl<I: ?Sized, W: RevWalk<I>> PeekableRevWalk<I, W> {
         self.peeked.as_ref()
     }
 
-    #[cfg(test)] // TODO
     pub fn next_if(
         &mut self,
         index: &I,