Rename prev -> _prev
Rename prev -> _prev

We needed to name our "next" method "_next" to avoid clashing with a
language keyword; for symmetry, let's prefix the "prev" method in the
same way.

mtimkovich/command-t,mtimkovich/command-t,lencioni/command-t,supriyantomaftuh/command-t,besarthoxhaj/command-t,vrkansagara/command-t,1234-/command-t,supriyantomaftuh/command-t,simichaels/dotfiles,wincent/command-t,wincent/command-t,vrkansagara/command-t,yrro/command-t,lencioni/command-t,simichaels/dotfiles,besarthoxhaj/command-t,wincent/command-t,1234-/command-t,yrro/command-t