Set &grepprg / &findfunc to git-grep / git-ls-files inside repo and fall back rg, ugrep or grep / fd, ... outside of it

grepfind.vim

" Set &grepprg to git-grep inside repo, fall back rg, ugrep or grep otherwise.
" Assumes a global ignore file `$XDG_CONFIG_HOME/grep/ignore`; create it by,
" say `touch ~/.config/grep/ignore` to ensure its existence.

augroup vimrcFindGrep
  autocmd!
augroup END

if has('unix') && executable('chrt') && executable('ionice')
  let s:scheduler = 'chrt --idle 0 ionice -c2 -n7 '
" " hard getting escaping right
" elseif has('win32')
"     let s:scheduler = (&shell =~? '\v(^|\\)cmd\.exe$' ? '' : 'cmd.exe ') .. 'start /B /LOW '
else
  let s:scheduler = ''
endif
let s:nul = ' 2> ' .. (has('win32') ? 'nul' : '/dev/null')

let s:smartcase = &ignorecase && &smartcase ? '--smart-case' : (&ignorecase ? '--ignore-case' : '--case-sensitive')

set grepformat=%f:%l:%c:%m,%f:%l:%m

if executable('rg')
  let &g:grepprg = s:scheduler .. 'rg --multiline --follow --no-heading --ignore-file=' .. expand('$XDG_CONFIG_HOME/grep/ignore') .. ' --with-filename --line-number --column  ' .. s:smartcase .. ' --color never $*' .. s:nul
elseif executable('ugrep')
  " From https://github.com/Genivia/ugrep#using-ugrep-within-vim
  let &g:grepprg = s:scheduler .. 'ugrep -R -n --ignore-binary --ungroup --tabs=1 --ignore-files=' .. expand('$XDG_CONFIG_HOME/grep/ignore') .. ' --column-number  ' .. s:smartcase .. ' --color=never --no-messages -- $*' .. s:nul
  setglobal grepformat=%f:%l:%c:%m,%f+%l+%c+%m,%-G%f\\\|%l\\\|%c\\\|%m
elseif executable('grep')
  let &g:grepprg = s:scheduler .. 'grep -rnHIsE $* --exclude=tags --exclude-dir=.git' .. s:nul
endif

if has('patch-9.1.0810') " = if exists('&findfunc')
  if executable('fd')
    let g:findcmd = 'fd --type file --full-path --ignore-file='..expand('$XDG_CONFIG_HOME/grep/ignore')..' --color never '..(has('win32') ? '--fixed-strings ' : '')..' ""'
  elseif executable('rg')
    let g:findcmd = 'rg --files --hidden --ignore-file='..expand('$XDG_CONFIG_HOME/grep/ignore')..' --no-ignore --color never --glob ""'
  elseif executable('ugrep')
    let g:findcmd = 'ugrep -Rl -I --ignore-files='..expand('$XDG_CONFIG_HOME/grep/ignore')..' --color=never -3 ""'
  else
    let g:findcmd = ''
    " if has('win32')
    "   let g:findcmd = 'dir . /s/b/a:-d-h'
    " elseif has('unix')
    "   let g:findcmd = 'find . -path "*/.git" -prune -o -type f -print'
    " endif
  endif

  " See :help live-grep
  func Find(arg, _)
    if empty(s:filescache)
      if empty(g:findcmd)
       let s:filescache = globpath('.', '**', 1, 1)
       call filter(s:filescache, '!isdirectory(v:val)')
       call map(s:filescache, "fnamemodify(v:val, ':.')")
      else
        let s:filescache =  systemlist(s:scheduler .. g:findcmd .. s:nul)
      endif
    endif
    return empty(a:arg) ? s:filescache : matchfuzzy(s:filescache, a:arg)
  endfunc
  autocmd vimrcFindGrep CmdlineEnter : let s:filescache = []

  set findfunc=Find
endif

if executable('git')
  function! s:outside_repo()
    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:grepprg = &g:grepprg
  let s:grepformat = &g:grepformat
  let s:git_grep = &ignorecase && &smartcase && executable('git-grep') ?
        \ 'git-grep' : (&ignorecase ? 'git grep --ignore-case' : 'git grep')
  let s:git_grep = s:scheduler .. s:git_grep .. ' --exclude-standard --column --line-number -I --untracked --extended-regexp --no-color $*' .. s:nul

  function! s:SetGrepPrg()
    if s:outside_repo()
      let &g:grepformat= s:grepformat
      let &g:grepprg = s:grepprg
    else
      setglobal grepformat=%f:%l:%c:%m
      let &g:grepprg = s:git_grep
    endif
  endfunction

  autocmd vimrcFindGrep VimEnter,DirChanged * call <SID>SetGrepPrg()

  if has('patch-9.1.0810') " = if exists('&findfunc')
    let s:findcmd = g:findcmd
    let s:git_ls = s:scheduler .. 'git ls-files . --exclude-from=' .. expand('$XDG_CONFIG_HOME/grep/ignore') .. ' --exclude-standard --cached --others' .. s:nul

    function! s:SetFindCmd()
      let g:findcmd = s:outside_repo() ? s:findcmd : s:git_ls
    endfunction

    autocmd vimrcFindGrep VimEnter,DirChanged * call <SID>SetFindCmd()
  endif
endif

if has('patch-9.1.1329') " = if exists('##CmdlineLeavePre')
  " See :help live-grep
  " Consider an Alias such as :Alias l LiveGrep
  command! -nargs=+ -complete=customlist,<SID>Grep LiveGrep call <SID>VisitFile()

  func s:Grep(arglead, cmdline, cursorpos)
    if match(&grepprg, '\$\*') == -1 | let &grepprg .= ' $*' | endif
    let cmd = substitute(&grepprg, '\$\*', shellescape(escape(a:arglead, '\')), '')
    return len(a:arglead) > 1 ? systemlist(cmd) : []
  endfunc

  func s:VisitFile()
    let item = getqflist(#{lines: [s:selected]}).items[0]
    let pos  = '[0,\ item.lnum,\ item.col,\ 0]'
    exe $':b +call\ setpos(".",\ {pos}) {item.bufnr}'
    call setbufvar(item.bufnr, '&buflisted', 1)
  endfunc

  " Automatically select the first item in the completion list when leaving the
  " command-line, and for `:Grep`, add the typed pattern to the command-line history:
  autocmd vimrcFindGrep CmdlineLeavePre :
        \ if get(cmdcomplete_info(), 'matches', []) != [] |
        \   let s:info = cmdcomplete_info() |
        \   if getcmdline() =~ '^\s*fin\%[d]\s' && s:info.selected == -1 |
        \     call setcmdline($'find {s:info.matches[0]}') |
        \   endif |
        \   if getcmdline() =~# '^\s*LiveGrep\s' |
        \     let s:selected = s:info.selected != -1
        \         ? s:info.matches[s:info.selected] : s:info.matches[0] |
        \     call setcmdline(s:info.cmdline_orig) |
        \   endif |
        \ endif

  " autocomplete matches for :find and :LiveGrep
  if has('patch-9.0.1576') " = if exists('*wildtrigger')
    augroup vimrcWildTrigger
      autocmd!
    augroup END

    autocmd vimrcFindGrep CmdlineChanged :
          \ if getcmdline() =~# '\v^\s*%(fin%[d]|LiveGrep)\s\S' && !exists('s:wildtrigger') | call s:wildtrigger() | endif

    function! s:wildtrigger() 
      let s:wildmode = &wildmode
      set wildmode=noselect,full
      autocmd vimrcWildTrigger CmdlineChanged : call wildtrigger()
      let s:wildtrigger = 1
      autocmd vimrcWildTrigger CmdlineLeavePre : ++once
            \ unlet! s:wildtrigger | let &wildmode = s:wildmode |
            \ autocmd! vimrcWildTrigger CmdlineChanged :
    endfunction
  endif
endif

See also https://gist.github.com/andlrc/c8e1a3b9c1ec5c761111ea0e49bda6c4

See this perl script gist for smart-case capable git-grep