dirstate-tree: Use HashMap instead of BTreeMap

BTreeMap has the advantage of its "natural" iteration order being the one we need
in the status algorithm. With HashMap however, iteration order is undefined so
we need to allocate a Vec and sort it explicitly.

Unfortunately many BTreeMap operations are slower than in HashMap, and skipping
that extra allocation and sort is not enough to compensate.

Switching to HashMap + sort makes `hg status` 17% faster in one test case,
as measure with hyperfine:

```
Benchmark  #1: ../hg2/hg status -R $REPO --config=experimental.dirstate-tree.in-memory=1
  Time (mean ± σ):     765.0 ms ±   8.8 ms    [User: 1.352 s, System: 0.747 s]
  Range (min … max):   751.8 ms … 778.7 ms    10 runs

Benchmark  #2: ./hg status -R $REPO --config=experimental.dirstate-tree.in-memory=1
  Time (mean ± σ):     651.8 ms ±   9.9 ms    [User: 1.251 s, System: 0.799 s]
  Range (min … max):   642.2 ms … 671.8 ms    10 runs

Summary
  './hg status -R $REPO --config=experimental.dirstate-tree.in-memory=1' ran
    1.17 ± 0.02 times faster than '../hg2/hg status -R $REPO --config=experimental.dirstate-tree.in-memory=1'
```

* ./hg is this revision
* ../hg2/hg is its parent
* $REPO is an old snapshot of mozilla-central

Differential Revision: https://phab.mercurial-scm.org/D10553

rust/hg-core/src/dirstate_tree/dirstate_map.rs

@@ -1,7 +1,7 b''
 use bytes_cast::BytesCast;
 use micro_timer::timed;
+use std::convert::TryInto;
 use std::path::PathBuf;
-use std::{collections::BTreeMap, convert::TryInto};
 
 use super::path_with_basename::WithBasename;
 use crate::dirstate::parsers::clear_ambiguous_mtime;
@@ -20,6 +20,7 b' use crate::DirstateMapError;'
 use crate::DirstateParents;
 use crate::DirstateStatus;
 use crate::EntryState;
+use crate::FastHashMap;
 use crate::PatternFileWarning;
 use crate::StateMapIter;
 use crate::StatusError;
@@ -43,7 +44,7 b' pub struct DirstateMap {'
 /// path, so comparing full paths gives the same result as comparing base
 /// names. However `BTreeMap` would waste time always re-comparing the same
 /// string prefix.
-pub(super) type ChildNodes = BTreeMap<WithBasename<HgPathBuf>, Node>;
+pub(super) type ChildNodes = FastHashMap<WithBasename<HgPathBuf>, Node>;
 
 /// Represents a file or a directory
 #[derive(Default)]
@@ -86,7 +87,7 b' impl DirstateMap {'
         Self {
             parents: None,
             dirty_parents: false,
-            root: ChildNodes::new(),
+            root: ChildNodes::default(),
             nodes_with_entry_count: 0,
             nodes_with_copy_source_count: 0,
         }

rust/hg-core/src/dirstate_tree/path_with_basename.rs

@@ -55,6 +55,12 b' impl<T: AsRef<HgPath>> Borrow<HgPath> fo'
     }
 }
 
+impl<T: AsRef<HgPath>> std::hash::Hash for WithBasename<T> {
+    fn hash<H: std::hash::Hasher>(&self, hasher: &mut H) {
+        self.base_name().hash(hasher)
+    }
+}
+
 impl<T: AsRef<HgPath> + PartialEq> PartialEq for WithBasename<T> {
     fn eq(&self, other: &Self) -> bool {
         self.base_name() == other.base_name()

rust/hg-core/src/dirstate_tree/status.rs

@@ -110,11 +110,11 b" impl<'tree, 'a> StatusCommon<'tree, 'a> "
 
         // `merge_join_by` requires both its input iterators to be sorted:
 
-        //
-        // * `BTreeMap` iterates according to keys’ ordering by definition
-
+        let mut dirstate_nodes: Vec<_> = dirstate_nodes.iter_mut().collect();
         // `sort_unstable_by_key` doesn’t allow keys borrowing from the value:
         // https://github.com/rust-lang/rust/issues/34162
+        dirstate_nodes
+            .sort_unstable_by(|(path1, _), (path2, _)| path1.cmp(path2));
         fs_entries.sort_unstable_by(|e1, e2| e1.base_name.cmp(&e2.base_name));
 
         itertools::merge_join_by(