gistfile1.vim

function! GitDiff()
  let l:myft = &filetype
  diffthis

" deal with submodules by temporarily changing to the file's dir
  cd %:p:h
  vnew | 0r !git show HEAD:./#:t
  norm 1G 
  exec 'set ft=' . l:myft
  set nomod ro 
" q to get rid
  map <buffer> q :quit<CR> :diffoff<CR>

  diffthis
  cd -
endfunction