def rev_sha1(rev)
    sha1 = nil
    execute "git rev-list -n 1 #{rev} --" do |out, e|
      sha1 = out.strip if !e
    end
    sha1
  end
def rev_sha1(rev)
    sha1 = nil
    execute "git rev-list -n 1 #{rev} --" do |out, e|
      sha1 = out.strip if !e
    end
    sha1
  end
returns the SHA-1 representation of rev