bug#37829: 27.0.50; Overlay behaviour changed without documentation.

[Zhu Zihao]

Reproduce step:

1. open a empty buffer, type "Lorem ipsum" in it, then goto point-min
2. Eval this code

(let ((ov (make-overlay (point-min) (1+ (point-at-eol)))))
  (overlay-put ov 'face 'mode-line))

In current version of Emacs, the mode line overlay will only cover the string
"Lorem ipsum", but in 26.2, the overlay will cover the hole line(every pixel in
line).

It may be a bug because no documentation or NEWS mentioned this change.

In GNU Emacs 27.0.50 (build 11, x86_64-pc-linux-gnu, GTK+ Version 3.24.12, 
cairo version 1.17.3)
 of 2019-10-20 built on archlinux
Repository revision: cea9577b7d6fcf01599afd48078f8ff1defb1297
Repository branch: makepkg
Windowing system distributor 'The X.Org Foundation', version 11.0.12005000
System Description: Arch Linux