Enhance git change detection by introducing relevant prefixes

- Updated the `git_changed_files_impl` and `collect_untracked_files` functions to accept a set of relevant prefixes, allowing for more targeted change detection.
- Modified the `collect_index_worktree_changes` function to filter changes based on the provided prefixes, improving efficiency in identifying tracked and untracked files.
- Added a new `known_paths` method to the `RojoTree` struct to facilitate the extraction of relevant directory paths.

These changes aim to optimize the detection of changes in the git repository by focusing on specified directory prefixes.

src/multimap.rs

@@ -39,6 +39,10 @@ impl<K: Hash + Eq, V: Eq> MultiMap<K, V> {
         bucket.push(v);
     }
 
+    pub fn keys(&self) -> hash_map::Keys<'_, K, Vec<V>> {
+        self.inner.keys()
+    }
+
     pub fn remove<Q: Borrow<K>, U: Borrow<V>>(&mut self, k: Q, v: U) -> Option<V> {
         let b = v.borrow();