merge-base, xdiff: zero out xpparam_t structures

merge-base, xdiff: zero out xpparam_t structures

xpparam_t structures are usually zero-initialized before their specific
fields are assigned to, but there are three locations in the tree where
that does not happen.  Add the missing memset() calls in order to make
initialization of xpparam_t structures consistent tree-wide and to
prevent stack garbage from being used as field values.

Signed-off-by: Michał Kępień <michal@isc.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>