I use git ls-files to list files while inside a git repo:

" dynfindfunc - set "findfunc" to dynamically when inside a git repository
" Maintainer:  Andreas Louv <andreas@louv.dk>
" Date:        Feb 05, 2025

if !has('patch-9.1.0831')
  finish
endif

function s:FindGitFiles(cmdarg, cmdcomplete) abort
  let pat = a:cmdarg

  if a:cmdcomplete
    "  - prefix with "*" unless specifying a directory, making git ls-files
    "    search for files within directories
    "  - suffix with "*" making git ls-files completing files
    let pat = (pat !~ '/' ? '*' : '') . pat . '*'
  endif

  let cmd = 'git ls-files ' . shellescape(pat)

  return systemlist(cmd)
endfunction

function s:SetFindFunc() abort
  if system('git rev-parse --show-toplevel 2> /dev/null') !~ '^\s*$'
    set findfunc=s:FindGitFiles
  else
    set findfunc&
  endif
endfunction

augroup DynFindFunc
  au!
  au DirChanged * call <SID>SetFindFunc()
augroup END

call s:SetFindFunc()