bug#49701: 28.0.50; Emacs no longer detects file modifications

[Dima Kogan]

Hi.

I'm running an emacs built from git today. The previous behavior of
detecting modified files doesn't work anymore. Recipe:

1. echo 123 > /tmp/whatever

2. emacs -Q /tmp/whatever
   (emacs now is looking at /tmp/whatever)

3. outside of emacs: echo 456 >> /tmp/whatever
   (i.e. we modified the file)

4. Back inside emacs, press any insertion key. Like 'a'

A this point, the non-bleeding-edge emacs would say see the
modification, and would ask the user if they want to revert or not. The
new emacs from git just inserts 'a' as if no outside modification
happened. I don't see anything in the NEWS that obviously changed this.

Thanks!