def list_tag(path = Dir.pwd)
      res = {}
      cg  = MiniGit::Capturing.new(path)
      unless (cg.tag :list => true).empty?
        # git show-ref --tags
        a = (cg.show_ref :tags => true).split("\n")
        res = Hash[ a.collect { |item| item.split(' refs/tags/') } ].invert
      end
      res
    end
def list_tag(path = Dir.pwd)
      res = {}
      cg  = MiniGit::Capturing.new(path)
      unless (cg.tag :list => true).empty?
        # git show-ref --tags
        a = (cg.show_ref :tags => true).split("\n")
        res = Hash[ a.collect { |item| item.split(' refs/tags/') } ].invert
      end
      res
    end
Get a hash table of tags under the format
  { <tag> => <commit> }