bug#23823: 25.0.95; Reset between highlight buffer/file comparisons

[Tino Calancha <f92capac@gmail.com>]

On Fri, 24 Jun 2016, Eli Zaretskii wrote:

> But the function's doc string clearly makes that expected behavior:
>
>  If the current buffer is visiting the file being compared against, it
>  also will have its differences highlighted.
>
> The only way I can interpret that "also" part is that the changes
> against the file are highlighted _in_addition_ to the changes tracked
> by the mode.  Your scenario just happens to produce a clash between
> these two sets of differences, and the result could be ambiguous.  But
> what we get in fact doesn't seem unreasonable to me.

You convinced me: there is no need to reset those highlighted differences.
If the user want to do that he/she could toggle eaasily the mode before 
calling the func.  Adding a prefix argument to reset may cause accidentaly
lost all the tracked differences.


The comentary of this lib helps to understand the design intentions.
For example when it says:
;; You can "age" different sets of changes by using
;; `highlight-changes-rotate-faces'.

Each time the buffer is saved, the new differences get a different face.

Following example shows the point:

emacs -Q -eval "(progn (with-temp-file \"/tmp/foo\" (insert \"Hi i am foo\n\")) (find-file \"/tmp/foo\") (highlight-changes-mode 1) (add-hook 'write-file-functions 'highlight-changes-rotate-faces nil t))"
M-: (progn (goto-char (point-max)) (insert "Hi foo, i am bar.\n") (highlight-compare-with-file "/tmp/foo")) RET y y
M-: (progn (goto-char (point-max)) (insert "Nice to meet you foo.\n") (highlight-compare-with-file "/tmp/foo")) RET y y
M-: (progn (goto-char (point-max)) (insert "Nice to meet you too, bar.\n") (highlight-compare-with-file "/tmp/foo")) RET y y

;; If we reset the buffer on each func call we will loose all this
;; psicodelic color structure :-S