Future-proof source for changes in xdemitconf_t

Future-proof source for changes in xdemitconf_t

The instances of xdemitconf_t were initialized member by member.
Instead, initialize them to all zero, so we do not have
to update those places each time we introduce a new member.

[jc: minimally fixed by getting rid of a new global]

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>