Make statusline redraw less aggressive

Mainly trying to avoid tainting `:verbose set readonly?`.

autoload/fugitive.vim

@@ -4214,7 +4214,9 @@ function! s:DoAutocmdChanged(dir) abort
   finally
     unlet! g:fugitive_event g:fugitive_result
     " Force statusline reload with the buffer's Git dir
-    let &l:ro = &l:ro
+    if dir isnot# FugitiveGitDir()
+      let &l:ro = &l:ro
+    endif
   endtry
   return ''
 endfunction