bug#23824: 25.0.95; Prevent compare one buffer with itself

[Tino Calancha <f92capac@gmail.com>]

On 06/23/2016 12:19 AM, Eli Zaretskii wrote:
>> From: Tino Calancha <f92capac@gmail.com>
>> Date: Wed, 22 Jun 2016 19:12:13 +0900 (JST)
>>
>> When the current buffer, buf-a, is visiting FILE-B, buf-b should
>> be a temporary buffer on sync with FILE-B.
>>
>> ./emacs -r -Q -eval '(progn (with-temp-file "/tmp/foo" (insert "foo"))
>> (find-file "/tmp/foo") (insert "bar"))'
>> M-: (highlight-compare-with-file "/tmp/foo") RET
>> n n ; Answer no to saving suggestions.
>> ;; Current buffer content different than /tmp/foo but no face
>> highlight-changes shown.
> I think we need first to establish what exactly is the semantic of
> this situation.  You are comparing a buffer with the file that the
> buffer visits.  The doc string of this function tries to say something
> about this situation:
>
>    If the current buffer is visiting the file being compared against, it
>    also will have its differences highlighted.  Otherwise, the file is
>    read in temporarily but the buffer is deleted.
>
> but I must confess that this is incomprehensible for me.  So I think
> we should first establish what that means, or what the code is trying
> to do.
  I understand what the doc means:  if the current buffer (buf-a) is 
visiting file-b,
then this func will perform a diff between buf-a and file-b.
* So, if  buf-a is modified, the command highlight you the differences 
with file-b, so
   let you decide if you want to save buf-a (overwritting file-b) or 
not.  It sounds useful.
* Current implementation doesn't match the doc string: even if buf-a is 
visiting file-b and
   modified, the func compare buf-a with buf-a, so that you never get 
nothing highlight
   in this case.

>> +                         (with-current-buffer buf-new
>> +                           (insert-file-contents-literally file-b)
> ??? Why insert-file-contents-literally?  That definitely sounds wrong.
>
> Thanks.

We can use: (insert-file-contents file-b)
it doesn't matter.  At the end, what this func does is comparing buffers
with ediff-diff-program so only the literal content would matter.