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>

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