zsh: don't set ZLE_RPROMPT_INDENT outside of tmux
zsh: don't set ZLE_RPROMPT_INDENT outside of tmux

For some reason, the terminal (both iTerm and Terminal.app) get confused
with this, eating the space to the right of PS1, which is not what we
want.

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent