Toggle visibility of highlighting due to Highlight Changes mode.
With a prefix argument ARG, enable Highlight Changes Visible mode
if ARG is positive, and disable it otherwise. If called from
Lisp, enable the mode if ARG is omitted or nil.
Highlight Changes Visible mode only has an effect when Highlight
Changes mode is on. When enabled, the changed text is displayed
in a distinctive face.
The default value can be customized with variable
`highlight-changes-visibility-initial-state'.
This command does not itself set highlight-changes mode.
(fn &optional ARG)