Don't gratuitously clobber quickfix in :Ggrep

plugin/fugitive.vim

@@ -1070,14 +1070,16 @@ function! s:Grep(cmd,bang,arg) abort
       if bufname(entry.bufnr) =~ ':'
         let entry.filename = s:repo().translate(bufname(entry.bufnr))
         unlet! entry.bufnr
+        let changed = 1
       elseif a:arg =~# '\%(^\| \)--cached\>'
         let entry.filename = s:repo().translate(':0:'.bufname(entry.bufnr))
         unlet! entry.bufnr
+        let changed = 1
       endif
     endfor
-    if a:cmd =~# '^l'
+    if a:cmd =~# '^l' && exists('changed')
       call setloclist(0, list, 'r')
-    else
+    elseif exists('changed')
       call setqflist(list, 'r')
     endif
     if !a:bang && !empty(list)