set vim path using git ls-tree

gitpath.vim

let s:nul = ' 2> ' .. (has('win32') ? 'nul' : '/dev/null')

function! s:outside_repo() abort
  if exists('*FugitiveGitDir')
    return empty(FugitiveGitDir())
  else
    " let outside_repo = empty(finddir('.git', getcwd().';')) && empty(findfile('.git', getcwd().';'))
    silent let repo = system('git rev-parse --is-inside-work-tree' .. s:nul .. ' 1>&2')
    return v:shell_error != 0
  endif
endfunction

let s:path = &g:path

function! s:SetPath() abort
  if s:outside_repo() | return | endif
  silent let paths = join(systemlist('git ls-tree -d --name-only -r HEAD' .. s:nul), ',')
  let &g:path = s:path .. ',' .. paths
endfunction
autocmd vimrc VimEnter,DirChanged * call <SID>SetPath()

See also @romainl 's trotten path gist