working_copy: don't build intermediate HashMap from proto file states

According to the doc, this is compatible with the map syntax.
https://protobuf.dev/programming-guides/proto3/#maps

This change means that the serialized file states are sorted by RepoPath,
so BTreeMap<RepoPath, _> can be reconstructed with fewer cache misses.

In my "linux" repo (watchman enabled):
 - jj-0: baseline
 - jj-1: this

    % hyperfine --sort command --warmup 3 --runs 10 -L bin jj-0,jj-1,jj-2 \
      "target/release-with-debug/{bin} -R ~/mirrors/linux status"
    Benchmark 1: target/release-with-debug/jj-0 -R ~/mirrors/linux status
      Time (mean ± σ):      1.034 s ±  0.020 s    [User: 0.881 s, System: 0.212 s]
      Range (min … max):    1.011 s …  1.068 s    10 runs

    Benchmark 2: target/release-with-debug/jj-1 -R ~/mirrors/linux status
      Time (mean ± σ):     849.3 ms ±  13.8 ms    [User: 710.7 ms, System: 199.3 ms]
      Range (min … max):   821.7 ms … 870.2 ms    10 runs

    Relative speed comparison
            1.32 ±  0.04  target/release-with-debug/jj-0 -R ~/mirrors/linux status
            1.08 ±  0.03  target/release-with-debug/jj-1 -R ~/mirrors/linux status

Cache-misses got reduced:

    % perf stat -e task-clock,cycles,instructions,cache-references,cache-misses \
      -- ./target/release-with-debug/jj-0 -R ~/mirrors/linux --no-pager status

              1,091.68 msec task-clock                       #    1.032 CPUs utilized
         4,179,596,978      cycles                           #    3.829 GHz
         6,166,231,489      instructions                     #    1.48  insn per cycle
           134,032,047      cache-references                 #  122.776 M/sec
            29,322,707      cache-misses                     #   21.88% of all cache refs

           1.057474164 seconds time elapsed

           0.897042000 seconds user
           0.194819000 seconds sys

    % perf stat -e task-clock,cycles,instructions,cache-references,cache-misses \
      -- ./target/release-with-debug/jj-1 -R ~/mirrors/linux --no-pager status

                927.05 msec task-clock                       #    1.083 CPUs utilized
         3,451,299,198      cycles                           #    3.723 GHz
         6,222,418,272      instructions                     #    1.80  insn per cycle
            98,499,363      cache-references                 #  106.251 M/sec
            11,998,523      cache-misses                     #   12.18% of all cache refs

           0.855938336 seconds time elapsed

           0.720568000 seconds user
           0.207924000 seconds sys

lib/src/local_working_copy.rs

@@ -15,7 +15,7 @@
 #![allow(missing_docs)]
 
 use std::any::Any;
-use std::collections::{BTreeMap, HashMap, HashSet};
+use std::collections::{BTreeMap, HashSet};
 use std::error::Error;
 use std::fs;
 use std::fs::{File, Metadata, OpenOptions};
@@ -195,12 +195,12 @@ fn file_state_to_proto(file_state: &FileState) -> crate::protos::working_copy::F
 }
 
 fn file_states_from_proto(
-    proto: &HashMap<String, crate::protos::working_copy::FileState>,
+    proto: &[crate::protos::working_copy::FileStateEntry],
 ) -> BTreeMap<RepoPath, FileState> {
     let mut file_states = BTreeMap::new();
-    for (path_str, proto_file_state) in proto {
-        let path = RepoPath::from_internal_string(path_str);
-        file_states.insert(path, file_state_from_proto(proto_file_state));
+    for entry in proto {
+        let path = RepoPath::from_internal_string(&entry.path);
+        file_states.insert(path, file_state_from_proto(entry.state.as_ref().unwrap()));
     }
     file_states
 }
@@ -465,12 +465,16 @@ impl TreeState {
             }
         }
 
-        for (file, file_state) in &self.file_states {
-            proto.file_states.insert(
-                file.to_internal_file_string(),
-                file_state_to_proto(file_state),
-            );
-        }
+        proto.file_states = self
+            .file_states
+            .iter()
+            .map(
+                |(path, state)| crate::protos::working_copy::FileStateEntry {
+                    path: path.to_internal_file_string(),
+                    state: Some(file_state_to_proto(state)),
+                },
+            )
+            .collect();
         let mut sparse_patterns = crate::protos::working_copy::SparsePatterns::default();
         for path in &self.sparse_patterns {
             sparse_patterns

lib/src/protos/working_copy.proto

@@ -32,6 +32,11 @@ message FileState {
   bytes conflict_id = 4 [deprecated = true];
 }
 
+message FileStateEntry {
+  string path = 1;
+  FileState state = 2;
+}
+
 message SparsePatterns {
   repeated string prefixes = 1;
 }
@@ -41,7 +46,7 @@ message TreeState {
   // Alternating positive and negative terms if there's a conflict, otherwise a
   // single (positive) value
   repeated bytes tree_ids = 5;
-  map<string, FileState> file_states = 2;
+  repeated FileStateEntry file_states = 2;
   SparsePatterns sparse_patterns = 3;
   WatchmanClock watchman_clock = 4;
 }

lib/src/protos/working_copy.rs

@@ -14,6 +14,14 @@ pub struct FileState {
 }
 #[allow(clippy::derive_partial_eq_without_eq)]
 #[derive(Clone, PartialEq, ::prost::Message)]
+pub struct FileStateEntry {
+    #[prost(string, tag = "1")]
+    pub path: ::prost::alloc::string::String,
+    #[prost(message, optional, tag = "2")]
+    pub state: ::core::option::Option<FileState>,
+}
+#[allow(clippy::derive_partial_eq_without_eq)]
+#[derive(Clone, PartialEq, ::prost::Message)]
 pub struct SparsePatterns {
     #[prost(string, repeated, tag = "1")]
     pub prefixes: ::prost::alloc::vec::Vec<::prost::alloc::string::String>,
@@ -27,11 +35,8 @@ pub struct TreeState {
     /// single (positive) value
     #[prost(bytes = "vec", repeated, tag = "5")]
     pub tree_ids: ::prost::alloc::vec::Vec<::prost::alloc::vec::Vec<u8>>,
-    #[prost(map = "string, message", tag = "2")]
-    pub file_states: ::std::collections::HashMap<
-        ::prost::alloc::string::String,
-        FileState,
-    >,
+    #[prost(message, repeated, tag = "2")]
+    pub file_states: ::prost::alloc::vec::Vec<FileStateEntry>,
     #[prost(message, optional, tag = "3")]
     pub sparse_patterns: ::core::option::Option<SparsePatterns>,
     #[prost(message, optional, tag = "4")]