if the tag is already checked out, to optimize the execution time.

I'm going to prepare a task depending on this tool, and I want that to
finish fast and output nothing when it's already up-to-date.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63883 b2dd03c8-39d4-4d8f-98ff-823fe69b080e