Add code below in your .vimrc to see the Git previous version of the current file with :GitPrevious.

gistfile1.txt

function! s:GitPrevious()
  	let _ = './' . expand('%')
	let filetype = &filetype

	execute ':silent! vsp ' . tempname()
	execute ':silent! 0r !git show HEAD~1:' . _
	execute ':silent! $d'
	execute ':silent! set nomodifiable'
	execute ':silent! set readonly'
	execute ':silent! set ft=' . filetype
	setlocal buftype=nowrite bufhidden=wipe nobuflisted noswapfile
endfunction

command! -nargs=0 GitPrevious call s:GitPrevious()