Take Tree.show_root into account when drawing guides

Rather than always start at the root, the code should start at the beginning
of the path.

See Textualize#2397.

src/textual/widgets/_tree.py

@@ -976,8 +976,8 @@ def _render_line(self, y: int, x1: int, x2: int, base_style: Style) -> Strip:
                 "tree--guides-selected", partial=True
             )
 
-            hover = self.root._hover
-            selected = self.root._selected and self.has_focus
+            hover = line.path[0]._hover
+            selected = line.path[0]._selected and self.has_focus
 
             def get_guides(style: Style) -> tuple[str, str, str, str]:
                 """Get the guide strings for a given style.