bug#21841: 25.0.50; auto-revert-mode broken when several files have modifications on disk

[Dmitry Gutov]

1. Turn on global-auto-revert-mode.
2. Open a few files that are different between different branches of the
Git repository you're working in (e.g. Emacs). Have at least 2 such files.
3. Switch from one branch to another. Notice that not every buffer has
updated its contents.
4. Switch to one such buffer, press SPC (or any other letter) - see the
"xxx changed on disk; really edit the buffer?" notification.

There's not much difference if I enable auto-revert-mode in each buffer
explicitly, instead of using global-auto-revert-mode.

In GNU Emacs 25.0.50.13 (x86_64-unknown-linux-gnu, GTK+ Version 3.16.7)
 of 2015-11-06
Repository revision: 267e0e80e1ad8c33c10ccaff77169a7aa759c163
Windowing system distributor 'The X.Org Foundation', version 11.0.11702000
System Description:     Ubuntu 15.10

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND DBUS GSETTINGS NOTIFY
GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB TOOLKIT_SCROLL_BARS
GTK3 X11