vcs.rb: for old ruby

* tool/vcs.rb (DebugSystem#system): pop option hash for old
  version ruby which does not support `system` options.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62217 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/vcs.rb

@@ -103,6 +103,7 @@ else
         exception = opts.fetch(:exception) {opts[:exception] = true}
       elsif opts
         exception = opts.delete(:exception) {true}
+        args.pop if opts.empty?
       end
       ret = super(*args)
       raise "Command failed with status (#$?): #{args[0]}" if exception and !ret