Add a git sign-off message using fugitive

gistfile1.txt

fun! AddGitSign()
  let idx=line('$')
  while idx > 0 && getline(idx) =~ '^#'
    let idx-=1
  endwhile
  let sign=['Signed-off-by: '.fugitive#buffer().repo().user()]
  if idx == 0 || getline(idx) != ''
    call insert(sign,'')
  endif
  call append(idx, sign)
endfun

aug MRGGit
au!
au FileType gitcommit map <buffer> <silent> <localleader>si :call AddGitSign()<CR>
au FileType gitcommit iabbrev <buffer> <silent> <expr> sign-- ((getline(line('.')-1)=~'^\s*$')?'':"\<CR>").
      \  'Signed-off-by: '.fugitive#buffer().repo().user()
aug END