versions: don't recreate the relative path on each iteration

Library/Homebrew/cmd/versions.rb

@@ -16,11 +16,12 @@ module Homebrew extend self
       if ARGV.include? '--compact'
         puts f.versions * " "
       else
+        relative_path = f.pretty_relative_path
         f.versions do |version, sha|
           print Tty.white.to_s
           print "#{version.to_s.ljust(8)} "
           print Tty.reset.to_s
-          puts "git checkout #{sha} #{f.pretty_relative_path}"
+          puts "git checkout #{sha} #{relative_path}"
         end
       end
     end