Always use absolute path for :Ggrep quickfix list

Ran into an issue where a running Vim (Neovim 0.9.5) consistently
treated a relative path not as the file in the current directory, but as
a different open buffer that matched the file name.  The relative path
was just for cosmetics; let's switch to absolute so Vim doesn't butcher
it.

autoload/fugitive.vim

@@ -5720,7 +5720,7 @@ function! s:GrepOptions(args, dir) abort
   let options = {'name_only': 0, 'name_count': 0, 'line_number': 0}
   let tree = s:Tree(a:dir)
   let prefix = empty(tree) ? fugitive#Find(':0:', a:dir) :
-        \ s:cpath(getcwd(), tree) ? '' : s:VimSlash(tree . '/')
+        \ s:VimSlash(tree . '/')
   let options.prefix = prefix
   for arg in a:args
     if arg ==# '--'