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This seems to be the correct thing to do, at least more in line with
what at least one other implementation does.  Anything using
gnome-desktop [0] effectively does the same, as
gnome_desktop_thumbnail_is_valid applies atol(3) to mtime for
comparison and time_t on GNU/Linux is a signed int.
[0] https://git.gnome.org/browse/gnome-desktop/
* lisp/image-dired.el (image-dired-create-thumb): Use floor here.