Ensure a node has no children before populating it

I'm struggling to recreate https://github.com/Textualize/frogmouth/issues/50
in a controlled way, but reviewing the code here makes me think that this is
a good idea anyway. While DirectoryTree should not end up in _populate_node
if a node has already been populated, it's also the case that it's an
all-or-nothing thing; it makes sense to clear out the children of the node
before populating it; at least in a belt-and-braces way.

src/textual/widgets/_directory_tree.py

@@ -271,6 +271,7 @@ class DirectoryTree(Tree[DirEntry]):
             node: The Tree node to populate.
             content: The collection of `Path` objects to populate the node with.
         """
+        node.remove_children()
         for path in content:
             node.add(
                 path.name,