Fix normalisedSelectedNodes function (#4920)

The `normalisedSelectedNodes` function in `files_controller.go` had a
bug where it didn't work correctly for the root item (`/`). This PR
fixes that.

We don't have any tests for this bug. Apparently, the functions that use
this (staging and discarding files) also work correctly when they work
on a directory and then on the contained file again, that's why nobody
has noticed yet. I briefly looked into adding unit tests for the
function, but it was more work than I was willing to put into this right
now.

pkg/gui/controllers/files_controller.go

@@ -1221,13 +1221,14 @@ func normalisedSelectedNodes(selectedNodes []*filetree.FileNode) []*filetree.Fil
 }
 
 func isDescendentOfSelectedNodes(node *filetree.FileNode, selectedNodes []*filetree.FileNode) bool {
+	nodePath := node.GetInternalPath()
+
 	for _, selectedNode := range selectedNodes {
 		if selectedNode.IsFile() {
 			continue
 		}
 
-		selectedNodePath := selectedNode.GetPath()
-		nodePath := node.GetPath()
+		selectedNodePath := selectedNode.GetInternalPath()
 
 		if strings.HasPrefix(nodePath, selectedNodePath+"/") {
 			return true