Fix dir node for hide-note-references=On

* info/nodes.c (info_get_node_tag_of_file_buffer):
New function, returning pointer into tags table.
(info_get_node_of_file_buffer): Use it.

* info/dir.c (build_dir_node): Use info_get_node_tag_of_file_buffer
along with info_node_of_tag_fast to load dir nodes without scanning
their contents.  Only call scan_node_contents once, on the composite
dir node.

When hide-note-references was On, the dir node was completely broken
due to parts of it being processed by scan_node_contents multiple
times.  Report from Aidan <aidanczhai@gmail.com>.