Properly compute depth and path for project panel entries (#19068)

Closes https://github.com/zed-industries/zed/issues/18939

This fixes incorrect width estimates and horizontal scrollbar glitches

Release Notes:

- Fixes horizontal scrollbar not scrolling enough for certain paths
([#18939](https://github.com/zed-industries/zed/issues/18939))

Co-authored-by: Piotr Osiewicz <piotr@zed.dev>

crates/project_panel/src/project_panel.rs

@@ -1911,11 +1911,9 @@ impl ProjectPanel {
                                     Some(Arc::<Path>::from(full_path.join(suffix)))
                                 })
                         })
+                        .or_else(|| entry.path.file_name().map(Path::new).map(Arc::from))
                         .unwrap_or_else(|| entry.path.clone());
-                    let depth = path
+                    let depth = path.components().count();
-                        .strip_prefix(worktree_abs_path)
-                        .map(|suffix| suffix.components().count())
-                        .unwrap_or_default();
                     (depth, path)
                 };
                 let width_estimate = item_width_estimate(