midx.c: replace `xcalloc()` with `CALLOC_ARRAY()`

Replace a direct invocation of Git's `xcalloc()` wrapper with the
`CALLOC_ARRAY()` macro instead.

The latter is preferred since it is more conventional in Git's codebase,
but also because it automatically picks the correct value for the record
size.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

midx.c

@@ -1912,9 +1912,11 @@ static int fill_included_packs_batch(struct repository *r,
 {
 	uint32_t i, packs_to_repack;
 	size_t total_size;
-	struct repack_info *pack_info = xcalloc(m->num_packs, sizeof(struct repack_info));
+	struct repack_info *pack_info;
 	int pack_kept_objects = 0;
 
+	CALLOC_ARRAY(pack_info, m->num_packs);
+
 	repo_config_get_bool(r, "repack.packkeptobjects", &pack_kept_objects);
 
 	for (i = 0; i < m->num_packs; i++) {