unpack-trees: special case read-tree debugging as internal usage

unpack-trees: special case read-tree debugging as internal usage

builtin/read-tree.c has some special functionality explicitly designed
for debugging unpack-trees.[ch].  Associated with that is two fields
that no other external caller would or should use.  Mark these as
internal to unpack-trees, but allow builtin/read-tree to read or write
them for this special case.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>