diff.c: rearrange xcalloc arguments

xcalloc() takes two arguments: the number of elements and their size.
diffstat_add() passes the arguments in reverse order, passing the
size of a diffstat_file*, followed by the number of diffstat_file* to
be allocated.

Rearrange them so they are in the correct order.
Signed-off-by: NBrian Gesiak <modocache@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

diff.c

@@ -1361,7 +1361,7 @@ static struct diffstat_file *diffstat_add(struct diffstat_t *diffstat,
 					  const char *name_b)
 {
 	struct diffstat_file *x;
-	x = xcalloc(sizeof (*x), 1);
+	x = xcalloc(1, sizeof(*x));
 	ALLOC_GROW(diffstat->files, diffstat->nr + 1, diffstat->alloc);
 	diffstat->files[diffstat->nr++] = x;
 	if (name_b) {