rust-dirstate: add support for nevermatcher

This is in case this ever comes up, it's very easy to support, so might as well
do it.

mercurial/dirstate.py

@@ -1248,6 +1248,7 b' class dirstate:'
             matchmod.exactmatcher,
             matchmod.includematcher,
             matchmod.intersectionmatcher,
+            matchmod.nevermatcher,
             matchmod.unionmatcher,
         )
 

rust/hg-core/src/matchers.rs

@@ -134,6 +134,31 b' impl Matcher for AlwaysMatcher {'
     }
 }
 
+/// Matches nothing.
+#[derive(Debug)]
+pub struct NeverMatcher;
+
+impl Matcher for NeverMatcher {
+    fn file_set(&self) -> Option<&HashSet<HgPathBuf>> {
+        None
+    }
+    fn exact_match(&self, _filename: &HgPath) -> bool {
+        false
+    }
+    fn matches(&self, _filename: &HgPath) -> bool {
+        false
+    }
+    fn visit_children_set(&self, _directory: &HgPath) -> VisitChildrenSet {
+        VisitChildrenSet::Empty
+    }
+    fn matches_everything(&self) -> bool {
+        false
+    }
+    fn is_exact(&self) -> bool {
+        true
+    }
+}
+
 /// Matches the input files exactly. They are interpreted as paths, not
 /// patterns.
 ///

rust/hg-cpython/src/dirstate/status.rs

@@ -15,7 +15,7 b' use cpython::{'
     PyResult, PyTuple, Python, PythonObject, ToPyObject,
 };
 use hg::dirstate::status::StatusPath;
-use hg::matchers::{Matcher, UnionMatcher, IntersectionMatcher};
+use hg::matchers::{IntersectionMatcher, Matcher, NeverMatcher, UnionMatcher};
 use hg::{
     matchers::{AlwaysMatcher, FileMatcher, IncludeMatcher},
     parse_pattern_syntax,
@@ -158,6 +158,7 b' fn extract_matcher('
 ) -> PyResult<Box<dyn Matcher + Sync>> {
     match matcher.get_type(py).name(py).borrow() {
         "alwaysmatcher" => Ok(Box::new(AlwaysMatcher)),
+        "nevermatcher" => Ok(Box::new(NeverMatcher)),
         "exactmatcher" => {
             let files = matcher.call_method(
                 py,