Improve readability of 'rake cache:clear' code

lib/tasks/cache.rake

 namespace :cache do
-  CLEAR_BATCH_SIZE = 1000
+  CLEAR_BATCH_SIZE = 1000 # The more the faster, but having too many can crash Ruby
   REDIS_SCAN_START_STOP = '0' # Magic value, see http://redis.io/commands/scan
 
   desc "GitLab | Clear redis cache"
   task :clear => :environment do
     redis_store = Rails.cache.instance_variable_get(:@data)
-    cursor = [REDIS_SCAN_START_STOP, []]
+    cursor = REDIS_SCAN_START_STOP
     loop do
-      cursor = redis_store.scan(
-        cursor.first,
+      cursor, keys = redis_store.scan(
+        cursor,
         match: "#{Gitlab::REDIS_CACHE_NAMESPACE}*", 
         count: CLEAR_BATCH_SIZE
       )
 
-      keys = cursor.last
       redis_store.del(*keys) if keys.any?
 
-      break if cursor.first == REDIS_SCAN_START_STOP
+      break if cursor == REDIS_SCAN_START_STOP
     end
   end
 end