vim-fzf-git-grep.vim

function! s:escape(path)
	return substitute(a:path, ' ', '\\ ', 'g')
endfunction

function! GitGrepHandler(line)
  let parts = split(a:line, ':')
  let [fn, lno] = parts[0 : 1]
  execute 'e '. s:escape(fn)
  execute lno
  normal! zz
endfunction

command! -nargs=+ Fgg call fzf#run({
  \ 'source': 'git grep -n -I --untracked -i "<args>"',
  \ 'sink': function('GitGrepHandler'),
  \ 'options': '+m',
  \ 'tmux_height': '60%'
\ })

To use it with vim-plug :

Plug 'junegunn/fzf', { 'dir': '~/.fzf', 'do': './install --all' }
Plug 'https://gist.github.com/adb258e1363a22e6e433.git',
    \ { 'dir': g:plug_home.'/vim-fzf-git-grep/plugin', 'rtp': '..' }