gitignore: make objects chain be more Arc friendly

This partially reverts changes in a9f489c "Switch to ignore crate for
gitignore handling." Since child ignore object no longer needs to access the
root to resolve the prefix path, it's simpler to store a matcher per node.

lib/src/gitignore.rs

@@ -16,7 +16,7 @@
 
 use std::path::PathBuf;
 use std::sync::Arc;
-use std::{fs, io};
+use std::{fs, io, iter};
 
 use ignore::gitignore;
 use thiserror::Error;
@@ -39,13 +39,15 @@ pub enum GitIgnoreError {
 /// Models the effective contents of multiple .gitignore files.
 #[derive(Debug)]
 pub struct GitIgnoreFile {
-    matchers: Vec<gitignore::Gitignore>,
+    parent: Option<Arc<GitIgnoreFile>>,
+    matcher: gitignore::Gitignore,
 }
 
 impl GitIgnoreFile {
     pub fn empty() -> Arc<GitIgnoreFile> {
         Arc::new(GitIgnoreFile {
-            matchers: Default::default(),
+            parent: None,
+            matcher: gitignore::Gitignore::empty(),
         })
     }
 
@@ -72,10 +74,12 @@ impl GitIgnoreFile {
             builder.add_line(None, line)?;
         }
         let matcher = builder.build()?;
-        let mut matchers = self.matchers.clone();
-        matchers.push(matcher);
-
-        Ok(Arc::new(GitIgnoreFile { matchers }))
+        let parent = if self.matcher.is_empty() {
+            self.parent.clone() // omit the empty root
+        } else {
+            Some(self.clone())
+        };
+        Ok(Arc::new(GitIgnoreFile { parent, matcher }))
     }
 
     /// Concatenates new `.gitignore` file at the `prefix` directory.
@@ -99,18 +103,17 @@ impl GitIgnoreFile {
     }
 
     fn matches_helper(&self, path: &str, is_dir: bool) -> bool {
-        self.matchers
-            .iter()
-            .rev()
-            .find_map(|matcher|
-                      // TODO: the documentation warns that
-                      // `matched_path_or_any_parents` is slower than `matched`;
-                      // ideally, we would switch to that.
-                      match matcher.matched_path_or_any_parents(path, is_dir) {
-                          ignore::Match::None => None,
-                          ignore::Match::Ignore(_) => Some(true),
-                          ignore::Match::Whitelist(_) => Some(false),
-                      })
+        iter::successors(Some(self), |file| file.parent.as_deref())
+            .find_map(|file| {
+                // TODO: the documentation warns that
+                // `matched_path_or_any_parents` is slower than `matched`;
+                // ideally, we would switch to that.
+                match file.matcher.matched_path_or_any_parents(path, is_dir) {
+                    ignore::Match::None => None,
+                    ignore::Match::Ignore(_) => Some(true),
+                    ignore::Match::Whitelist(_) => Some(false),
+                }
+            })
             .unwrap_or_default()
     }