bug#66381: 29.1; Auto-revert not polling files when notifications are enabled

[Daniel Jacobowitz]

I use a network filesystem which sometimes has to restart, e.g.  for
updates or when credentials expire. Normally, file notifications work
fine on this filesystem, but after a restart the old notifications will
never fire. Documentation says "By default, Auto Revert mode will poll
files for changes periodically even when file notifications are used." -
experimentally the file is never polled.

Ideally the notification would be recreated, but falling back to
polling would be
an improvement.

Reproduce from emacs -Q:

(global-auto-revert-mode t)
Open a file on the filesystem where notifications break
Append to the file externally -> emacs notices immediately
Restart the daemon, breaking notifications
Append to the file externally -> emacs never notices

This has been broken for a while. From auto-revert-handler:

         (revert
          (if buffer-file-name
              (and (or auto-revert-remote-files
                       (not (file-remote-p buffer-file-name)))
                   (or (not auto-revert-notify-watch-descriptor)
                       auto-revert-notify-modified-p)
                 ... eventually poll

Prior to bug#20943, there was a call to buffer-modified-p at the top
level that checked unconditionally whether the file had been modified.

Build:

In GNU Emacs 29.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.37,
 cairo version 1.16.0) of 2023-09-03, modified by Debian built on
 kokoro-ubuntu
System Description: Debian GNU/Linux rodete

There's a bunch of local bits in our Emacs build, but I'm ~mostly sure
they are unrelated
to this bug.

-- 
Thanks,
Daniel