dirstate-tree: Downgrade `&mut Node` to `&Node` in status and serialization

dirstate-tree: Downgrade `&mut Node` to `&Node` in status and serialization

Mutable access is not used, and upcoming changes will make it more costly
(with copy-on-write nodes that can be read from disk representation)

Differential Revision: https://phab.mercurial-scm.org/D10745

--- a/rust/hg-core/src/dirstate_tree/dirstate_map.rs	Wed May 19 13:15:00 2021 +0200
+++ b/rust/hg-core/src/dirstate_tree/dirstate_map.rs	Wed May 19 13:15:00 2021 +0200
@@ -68,9 +68,9 @@
     }
 
     pub(super) fn sorted<'tree>(
-        nodes: &'tree mut ChildNodes<'on_disk>,
-    ) -> Vec<(&'tree NodeKey<'on_disk>, &'tree mut Self)> {
-        let mut vec: Vec<_> = nodes.iter_mut().collect();
+        nodes: &'tree ChildNodes<'on_disk>,
+    ) -> Vec<(&'tree NodeKey<'on_disk>, &'tree Self)> {
+        let mut vec: Vec<_> = nodes.iter().collect();
         // `sort_unstable_by_key` doesn’t allow keys borrowing from the value:
         // https://github.com/rust-lang/rust/issues/34162
         vec.sort_unstable_by(|(path1, _), (path2, _)| path1.cmp(path2));

--- a/rust/hg-core/src/dirstate_tree/on_disk.rs	Wed May 19 13:15:00 2021 +0200
+++ b/rust/hg-core/src/dirstate_tree/on_disk.rs	Wed May 19 13:15:00 2021 +0200
@@ -258,7 +258,7 @@
 }
 
 fn write_nodes(
-    nodes: &mut dirstate_map::ChildNodes,
+    nodes: &dirstate_map::ChildNodes,
     out: &mut Vec<u8>,
 ) -> Result<ChildNodes, DirstateError> {
     // `dirstate_map::ChildNodes` is a `HashMap` with undefined iteration
@@ -269,7 +269,7 @@
     let mut on_disk_nodes = Vec::with_capacity(nodes.len());
     for (full_path, node) in nodes {
         on_disk_nodes.push(Node {
-            children: write_nodes(&mut node.children, out)?,
+            children: write_nodes(&node.children, out)?,
             tracked_descendants_count: node.tracked_descendants_count.into(),
             full_path: write_slice::<u8>(
                 full_path.full_path().as_bytes(),
@@ -287,7 +287,7 @@
                     len: 0.into(),
                 }
             },
-            entry: if let Some(entry) = &mut node.entry {
+            entry: if let Some(entry) = &node.entry {
                 OptEntry {
                     state: entry.state.into(),
                     mode: entry.mode.into(),

--- a/rust/hg-core/src/dirstate_tree/status.rs	Wed May 19 13:15:00 2021 +0200
+++ b/rust/hg-core/src/dirstate_tree/status.rs	Wed May 19 13:15:00 2021 +0200
@@ -56,7 +56,7 @@
     let has_ignored_ancestor = false;
     common.traverse_fs_directory_and_dirstate(
         has_ignored_ancestor,
-        &mut dmap.root,
+        &dmap.root,
         hg_path,
         &root_dir,
         is_at_repo_root,
@@ -93,7 +93,7 @@
     fn traverse_fs_directory_and_dirstate(
         &self,
         has_ignored_ancestor: bool,
-        dirstate_nodes: &'tree mut ChildNodes,
+        dirstate_nodes: &'tree ChildNodes,
         directory_hg_path: &'tree HgPath,
         directory_fs_path: &Path,
         is_at_repo_root: bool,
@@ -151,7 +151,7 @@
         &self,
         fs_entry: &DirEntry,
         hg_path: &'tree HgPath,
-        dirstate_node: &'tree mut Node,
+        dirstate_node: &'tree Node,
         has_ignored_ancestor: bool,
     ) {
         let file_type = fs_entry.metadata.file_type();
@@ -173,7 +173,7 @@
             let is_at_repo_root = false;
             self.traverse_fs_directory_and_dirstate(
                 is_ignored,
-                &mut dirstate_node.children,
+                &dirstate_node.children,
                 hg_path,
                 &fs_entry.full_path,
                 is_at_repo_root,
@@ -220,7 +220,7 @@
                 }
             }
 
-            for (child_hg_path, child_node) in &mut dirstate_node.children {
+            for (child_hg_path, child_node) in &dirstate_node.children {
                 self.traverse_dirstate_only(
                     child_hg_path.full_path(),
                     child_node,
@@ -278,10 +278,10 @@
     fn traverse_dirstate_only(
         &self,
         hg_path: &'tree HgPath,
-        dirstate_node: &'tree mut Node,
+        dirstate_node: &'tree Node,
     ) {
         self.mark_removed_or_deleted_if_file(hg_path, dirstate_node.state());
-        dirstate_node.children.par_iter_mut().for_each(
+        dirstate_node.children.par_iter().for_each(
             |(child_hg_path, child_node)| {
                 self.traverse_dirstate_only(
                     child_hg_path.full_path(),