rust-matchers: improve `Matcher` trait ergonomics

rust-matchers: improve `Matcher` trait ergonomics

`VisitChildrenSet` has no need to own the set, this will save allocations.

The `file_set` return type change is motivated by both ergonomics and... being
able to compile code.
The `AlwaysMatcher` does not store a `file_set`, which requires it to return an
owned `HashSet`, which in turn would change our return type to `Cow<&HgPath>`
(lifetimes omitted). This is both un-ergonomic and troublesome for more
complex lifetime issues (especially with the upcoming `FileMatcher` in the
following patch).

Differential Revision: https://phab.mercurial-scm.org/D7525

--- a/rust/hg-core/src/matchers.rs	Fri Nov 29 17:19:34 2019 +0100
+++ b/rust/hg-core/src/matchers.rs	Fri Nov 29 18:33:56 2019 +0100
@@ -10,21 +10,21 @@
 use crate::utils::hg_path::{HgPath, HgPathBuf};
 use std::collections::HashSet;
 
-pub enum VisitChildrenSet {
+pub enum VisitChildrenSet<'a> {
     /// Don't visit anything
     Empty,
     /// Only visit this directory
     This,
     /// Visit this directory and these subdirectories
     /// TODO Should we implement a `NonEmptyHashSet`?
-    Set(HashSet<HgPathBuf>),
+    Set(HashSet<&'a HgPath>),
     /// Visit this directory and all subdirectories
     Recursive,
 }
 
 pub trait Matcher {
     /// Explicitly listed files
-    fn file_set(&self) -> HashSet<&HgPath>;
+    fn file_set(&self) -> Option<&HashSet<&HgPath>>;
     /// Returns whether `filename` is in `file_set`
     fn exact_match(&self, filename: impl AsRef<HgPath>) -> bool;
     /// Returns whether `filename` is matched by this matcher
@@ -82,8 +82,8 @@
 pub struct AlwaysMatcher;
 
 impl Matcher for AlwaysMatcher {
-    fn file_set(&self) -> HashSet<&HgPath> {
-        HashSet::new()
+    fn file_set(&self) -> Option<&HashSet<&HgPath>> {
+        None
     }
     fn exact_match(&self, _filename: impl AsRef<HgPath>) -> bool {
         false