rhg: parallellize computation of [unsure_is_modified]

[unsure_is_modified] is called for every file for which we can't
determine its status based on its size and mtime alone.

In particular, this happens if the mtime of the file changes
without its contents changing.

Parallellizing this improves performance significantly when
we have many of these files.

Here's an example run (on a repo with ~400k files after dropping FS caches)

```
before:
real	0m53.901s
user	0m27.806s
sys	0m31.325s

after:
real	0m32.017s
user	0m34.277s
sys	1m26.250s
```

Another example run (a different FS):

```
before:
real	3m28.479s
user	0m31.800s
sys	0m25.324s

after:
real	0m29.751s
user	0m41.814s
sys	1m15.387s
```

rust/Cargo.lock

@@ -981,6 +981,7 b' dependencies = ['
  "lazy_static",
  "log",
  "micro-timer",
+ "rayon",
  "regex",
  "users",
  "which",

rust/hg-core/src/revlog/filelog.rs

@@ -17,18 +17,21 b' pub struct Filelog {'
 }
 
 impl Filelog {
-    pub fn open(repo: &Repo, file_path: &HgPath) -> Result<Self, HgError> {
+    pub fn open_vfs(
+        store_vfs: &crate::vfs::Vfs<'_>,
+        file_path: &HgPath,
+    ) -> Result<Self, HgError> {
         let index_path = store_path(file_path, b".i");
         let data_path = store_path(file_path, b".d");
-        let revlog = Revlog::open(
-            &repo.store_vfs(),
-            index_path,
-            Some(&data_path),
-            false,
-        )?;
+        let revlog =
+            Revlog::open(store_vfs, index_path, Some(&data_path), false)?;
         Ok(Self { revlog })
     }
 
+    pub fn open(repo: &Repo, file_path: &HgPath) -> Result<Self, HgError> {
+        Self::open_vfs(&repo.store_vfs(), file_path)
+    }
+
     /// The given node ID is that of the file as found in a filelog, not of a
     /// changeset.
     pub fn data_for_node(

rust/hg-core/src/sparse.rs

@@ -97,7 +97,7 b' pub(crate) fn parse_config('
         Includes,
         Excludes,
         None,
-    };
+    }
 
     let mut current = Current::None;
     let mut in_section = false;

rust/rhg/Cargo.toml

@@ -22,3 +22,4 b' env_logger = "0.9.0"'
 format-bytes = "0.3.0"
 users = "0.11.0"
 which = "4.2.5"
+rayon = "1.5.1"

rust/rhg/src/commands/status.rs

@@ -29,6 +29,7 b' use hg::StatusError;'
 use hg::StatusOptions;
 use hg::{self, narrow, sparse};
 use log::info;
+use rayon::prelude::*;
 use std::io;
 use std::path::PathBuf;
 
@@ -291,16 +292,31 b' pub fn run(invocation: &crate::CliInvoca'
             let manifest = repo.manifest_for_node(p1).map_err(|e| {
                 CommandError::from((e, &*format!("{:x}", p1.short())))
             })?;
-            for to_check in ds_status.unsure {
-                if unsure_is_modified(repo, &manifest, &to_check.path)? {
+            let working_directory_vfs = repo.working_directory_vfs();
+            let store_vfs = repo.store_vfs();
+            let res: Vec<_> = ds_status
+                .unsure
+                .into_par_iter()
+                .map(|to_check| {
+                    unsure_is_modified(
+                        working_directory_vfs,
+                        store_vfs,
+                        &manifest,
+                        &to_check.path,
+                    )
+                    .map(|modified| (to_check, modified))
+                })
+                .collect::<Result<_, _>>()?;
+            for (status_path, is_modified) in res.into_iter() {
+                if is_modified {
                     if display_states.modified {
-                        ds_status.modified.push(to_check);
+                        ds_status.modified.push(status_path);
                     }
                 } else {
                     if display_states.clean {
-                        ds_status.clean.push(to_check.clone());
+                        ds_status.clean.push(status_path.clone());
                     }
-                    fixup.push(to_check.path.into_owned())
+                    fixup.push(status_path.path.into_owned())
                 }
             }
         }
@@ -525,11 +541,12 b" impl DisplayStatusPaths<'_> {"
 /// This meant to be used for those that the dirstate cannot resolve, due
 /// to time resolution limits.
 fn unsure_is_modified(
-    repo: &Repo,
+    working_directory_vfs: hg::vfs::Vfs,
+    store_vfs: hg::vfs::Vfs,
     manifest: &Manifest,
     hg_path: &HgPath,
 ) -> Result<bool, HgError> {
-    let vfs = repo.working_directory_vfs();
+    let vfs = working_directory_vfs;
     let fs_path = hg_path_to_path_buf(hg_path).expect("HgPath conversion");
     let fs_metadata = vfs.symlink_metadata(&fs_path)?;
     let is_symlink = fs_metadata.file_type().is_symlink();
@@ -549,7 +566,7 b' fn unsure_is_modified('
     if entry.flags != fs_flags {
         return Ok(true);
     }
-    let filelog = repo.filelog(hg_path)?;
+    let filelog = hg::filelog::Filelog::open_vfs(&store_vfs, hg_path)?;
     let fs_len = fs_metadata.len();
     let file_node = entry.node_id()?;
     let filelog_entry = filelog.entry_for_node(file_node).map_err(|_| {