Cluster: don't send -ASK to MIGRATE.

For non existing keys, we don't want to send -ASK redirections to
MIGRATE, since when moving slots from the migrating node to the
importing node, we want just to ignore keys that are no longer there.
They may be expired or deleted between the GETKEYSINSLOT call and the
MIGRATE call. Otherwise this causes an error during migrations with
redis-trib (or equivalent cluster management tools).

src/cluster.c

@@ -4869,7 +4869,7 @@ void readwriteCommand(client *c) {
  * CLUSTER_REDIR_CROSS_SLOT if the request contains multiple keys that
  * don't belong to the same hash slot.
  *
- * CLUSTER_REDIR_UNSTABLE if the request contains mutliple keys
+ * CLUSTER_REDIR_UNSTABLE if the request contains multiple keys
  * belonging to the same slot, but the slot is not stable (in migration or
  * importing state, likely because a resharding is in progress).
  *
@@ -4993,8 +4993,10 @@ clusterNode *getNodeByQuery(client *c, struct redisCommand *cmd, robj **argv, in
      * Then if we have all the keys. */
 
     /* If we don't have all the keys and we are migrating the slot, send
-     * an ASK redirection. */
-    if (migrating_slot && missing_keys) {
+     * an ASK redirection. With the exception of the MIGRATE command, that
+     * should just ignore non existing keys when moving keys from a node
+     * to another. */
+    if (migrating_slot && missing_keys && cmd->proc != migrateCommand) {
         if (error_code) *error_code = CLUSTER_REDIR_ASK;
         return server.cluster->migrating_slots_to[slot];
     }