magit-revert-buffers: revert buffers in emacs 24.4

In Emacs 24.4, when `auto-revert-use-notify` is t, but auto-revert is
not enabled for current buffer (auto-revert-notify-modified-p remains
nil), `auto-revert-handler` will not revert the buffer. Setting
`auto-revert-mode` to t for buffer to be reverted will make
`auto-revert-handler` effective.