rust-matchers: fix quadratic complexity in `FileMatcher`

Concretely, this command:

```
$ echo hg up -r <nodeid>; time hg revert dir1 dir2 -r <othernode> --debug
hg up -r <nodeid>

real	0m14.690s
user	0m14.766s
sys	0m5.430s
```

was much slower despite using 16 cores before this change.
The approach taken here is the same one used in match.py, in exactmatcher.

This changeset was originally written by Valentin Gatien-Baron in a private
repository.
I have redacted the commit message and did a minor clean up of the code.

rust/hg-core/src/matchers.rs

@@ -7,6 +7,9 b''
 
 //! Structs and types for matching files and directories.
 
+use format_bytes::format_bytes;
+use once_cell::sync::OnceCell;
+
 use crate::{
     dirstate::dirs_multiset::DirsChildrenMultiset,
     filepatterns::{
@@ -23,11 +26,11 b' use crate::{'
 
 use crate::dirstate::status::IgnoreFnType;
 use crate::filepatterns::normalize_path_bytes;
-use std::borrow::ToOwned;
 use std::collections::HashSet;
 use std::fmt::{Display, Error, Formatter};
 use std::ops::Deref;
 use std::path::{Path, PathBuf};
+use std::{borrow::ToOwned, collections::BTreeSet};
 
 #[derive(Debug, PartialEq)]
 pub enum VisitChildrenSet {
@@ -173,6 +176,7 b' impl Matcher for NeverMatcher {'
 pub struct FileMatcher {
     files: HashSet<HgPathBuf>,
     dirs: DirsMultiset,
+    sorted_visitchildrenset_candidates: OnceCell<BTreeSet<HgPathBuf>>,
 }
 
 impl FileMatcher {
@@ -181,6 +185,7 b' impl FileMatcher {'
         Ok(Self {
             files: HashSet::from_iter(files.into_iter()),
             dirs,
+            sorted_visitchildrenset_candidates: OnceCell::new(),
         })
     }
     fn inner_matches(&self, filename: &HgPath) -> bool {
@@ -199,30 +204,34 b' impl Matcher for FileMatcher {'
         self.inner_matches(filename)
     }
     fn visit_children_set(&self, directory: &HgPath) -> VisitChildrenSet {
-        if self.files.is_empty() || !self.dirs.contains(&directory) {
+        if self.files.is_empty() || !self.dirs.contains(directory) {
             return VisitChildrenSet::Empty;
         }
-        let mut candidates: HashSet<HgPathBuf> =
-            self.dirs.iter().cloned().collect();
 
-        candidates.extend(self.files.iter().cloned());
-        candidates.remove(HgPath::new(b""));
-
-        if !directory.as_ref().is_empty() {
-            let directory = [directory.as_ref().as_bytes(), b"/"].concat();
-            candidates = candidates
-                .iter()
-                .filter_map(|c| {
-                    if c.as_bytes().starts_with(&directory) {
-                        Some(HgPathBuf::from_bytes(
-                            &c.as_bytes()[directory.len()..],
-                        ))
-                    } else {
-                        None
-                    }
-                })
-                .collect();
-        }
+        let compute_candidates = || -> BTreeSet<HgPathBuf> {
+            let mut candidates: BTreeSet<HgPathBuf> =
+                self.dirs.iter().cloned().collect();
+            candidates.extend(self.files.iter().cloned());
+            candidates.remove(HgPath::new(b""));
+            candidates
+        };
+        let candidates =
+            if directory.as_ref().is_empty() {
+                compute_candidates()
+            } else {
+                let sorted_candidates = self
+                    .sorted_visitchildrenset_candidates
+                    .get_or_init(compute_candidates);
+                let directory_bytes = directory.as_ref().as_bytes();
+                let start: HgPathBuf =
+                    format_bytes!(b"{}/", directory_bytes).into();
+                let start_len = start.len();
+                // `0` sorts after `/`
+                let end = format_bytes!(b"{}0", directory_bytes).into();
+                BTreeSet::from_iter(sorted_candidates.range(start..end).map(
+                    |c| HgPathBuf::from_bytes(&c.as_bytes()[start_len..]),
+                ))
+            };
 
         // `self.dirs` includes all of the directories, recursively, so if
         // we're attempting to match 'foo/bar/baz.txt', it'll have '', 'foo',