Add a Tig key binding for "git rebase -i"

ref.
  sue445/dotfiles/_tigrc commits on 2013-04-08 and 2013-04-25
  https://github.com/sue445/dotfiles/commit/81a236a0b04a323044496511dfcceaebf2bdee85
  https://github.com/sue445/dotfiles/commit/d4df6a8827a5a3df2363f3c7379e996d4dfeb57f