index-format.txt: clarify what is "invalid"

A cache-tree entry with a negative entry count is considered invalid by the current Git; it records that we do not know the object name of a tree that would result by writing the directory covered by the cache-tree as a tree object. Clarify that any entry with a negative entry count is invalid, but the implementations must write -1 there. This way, we can later decide to allow writers to use negative values other than -1 to encode optional information on such invalidated entries without harming interoperability; we do not know what will be encoded and how, so we keep these other negative values as reserved for now. Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>