あうあう
$ git checkout use-ruby31
Switched to branch 'use-ruby31'
Your branch and 'origin/use-ruby31' have diverged,
and have 1 and 104 different commits each, respectively.
(use "git pull" to merge the remote branch into yours)
たぶんoriginが正しい！うりゃ。
$ git reset --hard origin/use-ruby31
HEAD is now at 2e7a090f6 Merge branch 'master' into use-ruby31