refactor(nix-compat/nar/reader): reuse prev_name allocation

We reuse the prev_name allocation for Entry, instead of allocating and
returning a separate Vec.

We encode the `prev_name: None` case as an empty vector, since we don't
allow empty names anyway, and the sorting is equivalent.

Change-Id: I975b37ff873805f5ff099bc82128706891052247
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11607
Reviewed-by: Brian Olsen <me@griff.name>
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/tools/crunch-v2/src/main.rs

@@ -147,7 +147,7 @@ fn ingest(node: nar::Node, name: Vec<u8>, avg_chunk_size: u32) -> Result<proto::
             let mut symlinks = vec![];
 
             while let Some(node) = reader.next()? {
-                match ingest(node.node, node.name, avg_chunk_size)? {
+                match ingest(node.node, node.name.to_owned(), avg_chunk_size)? {
                     proto::path::Node::Directory(node) => {
                         directories.push(node);
                     }