Teach notes code to properly preserve non-notes in the notes tree

Teach notes code to properly preserve non-notes in the notes tree

The note tree structure allows for non-note entries to coexist with note
entries in a notes tree. Although we certainly expect there to be very
few non-notes in a notes tree, we should still support them to a certain
degree.

This patch teaches the notes code to preserve non-notes when updating the
notes tree with write_notes_tree(). Non-notes are not affected by fanout
restructuring.

For non-notes to be handled correctly, we can no longer allow subtree
entries that do not match the fanout structure produced by the notes code
itself. This means that fanouts like 4/36, 6/34, 8/32, 4/4/32, etc. are
no longer recognized as note subtrees; only 2-based fanouts are allowed
(2/38, 2/2/36, 2/2/2/34, etc.). Since the notes code has never at any point
_produced_ non-2-based fanouts, it is highly unlikely that this change will
cause problems for anyone.

The patch also adds some tests verifying the correct handling of non-notes
in a notes tree.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>