bug#39427: 26.3; whitespace-mode doesn't track fill-column changes

[Trey Harris <treyharris@gmail.com>]

1. In *scratch*, toggle whitespace-mode (M-x whitespace-mode RET)

2. Do (C-u 65 x) to get 65 characters, then continue typing into the
line. When you reach 70 (or the value of fill-column), the line will
change face to the long line variant.

3. Do set-fill-column (C-x f) and accept the current value matching 
point's current column. Then type "some more".

**Expected behavior:** the long-line face variant should go away when you
increase fill-column, until you begin typing "some-more".

**Actual behaviOor:** the face does not change.

4. Toggle whitespace-mode off and back on (M-x whitespace-mode RET M-xx
whitespace-mode RET)u

5. Delete "some-more" and press DEL again to get the line under a width
of fill-column. Note that the face changes, as a toggle of
whitespace-mode caused it to re-read fill-column's value.

When whitespace-mode is enabled and whitespace-line-column is set to nil
(meaning track fill-column,) it should track changes to fill-column.

But in fact, whitespace-mode doesn't even track changes to
whitespace-line-column; if you set it to, say, 200 at step 3 above, the
line does not re-face without toggling whitespace-mode, either.
---

In GNU Emacs 26.3 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.22.30)
 of 2019-09-16 built on lcy01-amd64-030
System Description:	Ubuntu 18.04.3 LTS

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