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>