EMFolderTreeModel: Fix subscribing to a folder with children.

Unsubscribe from a folder with children, then re-subscribe.
The children will no longer be reachable in an EMFolderTree.

This is because we were calling em_folder_tree_model_set_folder_info()
claiming we have complete folder subtree info, when in fact we don't.

(cherry picked from commit b0106cc84284bad5d8d84192ff8aab54bb06ae1d)

Conflicts:
	mail/em-folder-tree-model.c