xdiff: introduce xdl_calloc

xdiff: introduce xdl_calloc

In preparation for introducing XDL_CALLOC_ARRAY() use calloc() to
obtain zeroed out memory rather than malloc() followed by memset(). To
try and keep the lines a reasonable length this commit also stops
casting the pointer returned by calloc() as this is unnecessary.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>