convert trivial cases to ALLOC_ARRAY

Each of these cases can be converted to use ALLOC_ARRAY or
REALLOC_ARRAY, which has two advantages:

  1. It automatically checks the array-size multiplication
     for overflow.

  2. It always uses sizeof(*array) for the element-size,
     so that it can never go out of sync with the declared
     type of the array.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -903,7 +903,7 @@ static int remove_redundant(struct commit **array, int cnt)
 
 	work = xcalloc(cnt, sizeof(*work));
 	redundant = xcalloc(cnt, 1);
-	filled_index = xmalloc(sizeof(*filled_index) * (cnt - 1));
+	ALLOC_ARRAY(filled_index, cnt - 1);
 
 	for (i = 0; i < cnt; i++)
 		parse_commit(array[i]);