Since in-memory index entries are allocated individually now, the
variable slack at the end meant to provide an eight byte alignment
is not needed anymore. Have a single NUL instead. This saves zero
to seven bytes for an entry, depending on its filename length.
Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx> Signed-off-by: Junio C Hamano <gitster@pobox.com>