index: make topo_order() return commit ids instead of index entries

`IndexEntry` is specific to the default index store; we don't want it
in the interface.

lib/src/default_index_store.rs

@@ -724,7 +724,7 @@ impl Index for MutableIndexImpl {
         CompositeIndex(self).heads(candidates)
     }
 
-    fn topo_order(&self, input: &mut dyn Iterator<Item = &CommitId>) -> Vec<IndexEntry> {
+    fn topo_order(&self, input: &mut dyn Iterator<Item = &CommitId>) -> Vec<CommitId> {
         CompositeIndex(self).topo_order(input)
     }
 
@@ -1109,10 +1109,10 @@ impl<'a> CompositeIndex<'a> {
     }
 
     /// Parents before children
-    fn topo_order(&self, input: &mut dyn Iterator<Item = &CommitId>) -> Vec<IndexEntry<'a>> {
-        let mut entries_by_generation = input.map(|id| self.entry_by_id(id).unwrap()).collect_vec();
-        entries_by_generation.sort_unstable_by_key(|e| e.pos);
-        entries_by_generation
+    fn topo_order(&self, input: &mut dyn Iterator<Item = &CommitId>) -> Vec<CommitId> {
+        let mut ids = input.cloned().collect_vec();
+        ids.sort_by_cached_key(|id| self.commit_id_to_pos(id).unwrap());
+        ids
     }
 }
 
@@ -1802,7 +1802,7 @@ impl Index for ReadonlyIndexImpl {
         CompositeIndex(self).heads(candidates)
     }
 
-    fn topo_order(&self, input: &mut dyn Iterator<Item = &CommitId>) -> Vec<IndexEntry> {
+    fn topo_order(&self, input: &mut dyn Iterator<Item = &CommitId>) -> Vec<CommitId> {
         CompositeIndex(self).topo_order(input)
     }
 

lib/src/index.rs

@@ -67,7 +67,7 @@ pub trait Index: Send + Sync {
     fn heads(&self, candidates: &mut dyn Iterator<Item = &CommitId>) -> Vec<CommitId>;
 
     /// Parents before children
-    fn topo_order(&self, input: &mut dyn Iterator<Item = &CommitId>) -> Vec<IndexEntry>;
+    fn topo_order(&self, input: &mut dyn Iterator<Item = &CommitId>) -> Vec<CommitId>;
 
     // TODO: It's weird that we pass in the repo here since the repo is a
     // higher-level concept. We should probably pass in the view and store

src/commands/mod.rs

@@ -1902,7 +1902,6 @@ fn cmd_duplicate(
         .index()
         .topo_order(&mut to_duplicate.iter().map(|c| c.id()))
         .into_iter()
-        .map(|index_entry| index_entry.commit_id())
     {
         // Topological order ensures that any parents of `original_commit` are
         // either not in `to_duplicate` or were already duplicated.