Never jump to invalid entry after quickfix command

In particular, if the first :Ggrep result is a binary file, skip over
it.  I'm also leaning strongly towards not filling in the filename field
at all for binary files, but I'll address that separately.

autoload/fugitive.vim

@@ -1576,11 +1576,15 @@ function! s:QuickfixStream(nr, event, title, cmd, first, mods, callback, ...) ab
   call s:QuickfixSet(a:nr, buffer, 'a')
 
   exe s:DoAutocmd('QuickFixCmdPost ' . event)
-  if a:first && len(s:QuickfixGet(a:nr))
+  if a:first
-    return (a:nr < 0 ? 'cfirst' : 'lfirst')
+    let list = s:QuickfixGet(a:nr)
-  else
+    for index in range(len(list))
-    return 'exe'
+      if list[index].valid
+        return (index+1) . (a:nr < 0 ? 'cfirst' : 'lfirst')
       endif
+    endfor
+  endif
+  return 'exe'
 endfunction
 
 function! fugitive#Cwindow() abort