HackStudio: Fix opened files appearing empty (#5480)

When files where placed in outside of the project root, they would
appear empty because the label in the tree would differ from the
actual file path relative to the root.

Fixes #5471.

Userland/DevTools/HackStudio/HackStudioWidget.cpp

@@ -695,8 +695,8 @@ void HackStudioWidget::create_project_tree_view(GUI::Widget& parent)
     };
 
     m_project_tree_view->on_activation = [this](auto& index) {
-        auto filename = index.data().as_string();
-        open_file(filename);
+        auto full_path_to_file = m_project->model().full_path(index);
+        open_file(full_path_to_file);
     };
 }