Clean up RelPath API (#38912)

Consolidate constructors and accessors.

Release Notes:

- N/A

---------

Co-authored-by: Cole Miller <cole@zed.dev>

crates/project/src/git_store/git_traversal.rs

@@ -332,7 +332,7 @@ mod tests {
 
         let traversal = GitTraversal::new(
             &repo_snapshots,
-            worktree_snapshot.traverse_from_path(true, false, true, RelPath::new("x").unwrap()),
+            worktree_snapshot.traverse_from_path(true, false, true, RelPath::unix("x").unwrap()),
         );
         let entries = traversal
             .map(|entry| (entry.path.clone(), entry.git_summary))