Change fzf#vim#grep to rely on temporary FZF_DEFAULT_COMMAND

Related #907
Also junegunn/fzf#1750

function! RipgrepFzf(query, fullscreen)
  let command_fmt = 'rg --column --line-number --no-heading --color=always --smart-case %s || true'
  let initial_command = printf(command_fmt, shellescape(a:query))
  let reload_command = printf(command_fmt, '{q}')
  let options = {'options': ['--phony', '--query', a:query, '--bind', 'change:reload:'.reload_command]}
  call fzf#vim#grep(initial_command, 1, options, a:fullscreen)
endfunction

command! -nargs=* -bang RF call RipgrepFzf(<q-args>, <bang>0)

Author: junegunn

autoload/fzf/vim.vim

@@ -695,7 +695,7 @@ function! fzf#vim#ag_raw(command_suffix, ...)
   return call('fzf#vim#grep', extend(['ag --nogroup --column --color '.a:command_suffix, 1], a:000))
 endfunction
 
-" command, with_column, [options]
+" command (string), with_column (0/1), [options (dict)], [fullscreen (0/1)]
 function! fzf#vim#grep(grep_command, with_column, ...)
   let words = []
   for word in split(a:grep_command)
@@ -708,7 +708,6 @@ function! fzf#vim#grep(grep_command, with_column, ...)
   let name    = join(words, '-')
   let capname = join(map(words, 'toupper(v:val[0]).v:val[1:]'), '')
   let opts = {
-  \ 'source':  a:grep_command,
   \ 'column':  a:with_column,
   \ 'options': ['--ansi', '--prompt', capname.'> ',
   \             '--multi', '--bind', 'alt-a:select-all,alt-d:deselect-all',
@@ -718,7 +717,13 @@ function! fzf#vim#grep(grep_command, with_column, ...)
     return s:ag_handler(a:lines, self.column)
   endfunction
   let opts['sink*'] = remove(opts, 'sink')
-  return s:fzf(name, opts, a:000)
+  try
+    let prev_default_command = $FZF_DEFAULT_COMMAND
+    let $FZF_DEFAULT_COMMAND = a:grep_command
+    return s:fzf(name, opts, a:000)
+  finally
+    let $FZF_DEFAULT_COMMAND = prev_default_command
+  endtry
 endfunction
 
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