vcs.rb: old version cannot refine modules

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62060 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/vcs.rb

@@ -86,7 +86,11 @@ else
         super
       end
     end
-    refine Kernel do
+  ensure
+    $VERBOSE = verbose unless verbose.nil?
+  end
+  using DebugPOpen
+  module DebugSystem
     def system(*args, exception: true, **opts)
       STDERR.puts [*args, **opts].inspect if $DEBUG
       ret = super(*args, **opts, exception: exception)
@@ -94,10 +98,9 @@ else
       ret
     end
   end
-  ensure
-    $VERBOSE = verbose unless verbose.nil?
+  module Kernel
+    prepend(DebugSystem)
   end
-  using DebugPOpen
 end
 
 class VCS