perf(fmt): avoid relative resolution on cache hits (#199)

Author: chenjiahan

packages/rstack/src/fmt/discoverPaths.ts

@@ -153,15 +153,14 @@ class GitIgnoreMatcher {
     return loading;
   }
 
-  #isDirectoryIgnored(
-    directoryPath: string,
-    relativePath = path.relative(this.#rootPath, directoryPath),
-  ): boolean {
+  #isDirectoryIgnored(directoryPath: string, relativePath?: string): boolean {
     const cached = this.#ignoredDirectories.get(directoryPath);
     if (cached !== undefined) {
       return cached;
     }
 
+    relativePath ??= path.relative(this.#rootPath, directoryPath);
+
     // Git cannot re-include a path below an ignored directory.
     const parentPath = path.dirname(directoryPath);
     const ignored =