Replace custom memory growth allocator with ALLOC_GROW

The ALLOC_GROW macro is a shorter way to implement an array that
grows upon demand as additional items are added to it.  We have
mostly standardized upon its use within git and transport.c is
not an exception.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

transport.c

@@ -474,13 +474,11 @@ struct ref *transport_get_remote_refs(struct transport *transport)
 	return transport->remote_refs;
 }
 
-#define PACK_HEADS_CHUNK_COUNT 256
-
 int transport_fetch_refs(struct transport *transport, struct ref *refs)
 {
 	int i;
-	int nr_heads = 0;
-	char **heads = xmalloc(PACK_HEADS_CHUNK_COUNT * sizeof(char *));
+	int nr_heads = 0, nr_alloc = 0;
+	char **heads = NULL;
 	struct ref *rm;
 	int use_objs = !transport->ops->fetch_refs;
 
@@ -488,15 +486,12 @@ int transport_fetch_refs(struct transport *transport, struct ref *refs)
 		if (rm->peer_ref &&
 		    !hashcmp(rm->peer_ref->old_sha1, rm->old_sha1))
 			continue;
+		ALLOC_GROW(heads, nr_heads + 1, nr_alloc);
 		if (use_objs) {
 			heads[nr_heads++] = xstrdup(sha1_to_hex(rm->old_sha1));
 		} else {
 			heads[nr_heads++] = xstrdup(rm->name);
 		}
-		if (nr_heads % PACK_HEADS_CHUNK_COUNT == 0)
-			heads = xrealloc(heads,
-					 (nr_heads + PACK_HEADS_CHUNK_COUNT) *
-					 sizeof(char *));
 	}
 
 	if (use_objs) {