bug#37395: [PATCH] diff-mode.el: take into account patch separators

[Konstantin Kharlamov <hi-angel@yandex.ru>]

On Пт, сен 13, 2019 at 09:14, Eli Zaretskii <eliz@gnu.org> wrote:
>>  From: Konstantin Kharlamov <Hi-Angel@yandex.ru>
>>  Date: Fri, 13 Sep 2019 00:34:45 +0300
>> 
>>  * lisp/vc/diff-mode.el (diff-goto-line-before-patch-separator): an
>>  inline function to check if prev. line was git-format-patch 
>> separator,
>>  in which case go there.
>>  (diff-end-of-hunk): make use of 
>> (diff-goto-line-before-patch-separator)
> 
> The descriptions of changes should start with a capital letter.  Also,
> your lines in the commit log message are too long, they should not
> exceed 61 characters (because in the release tarball we create a
> ChangeLog file from them, which prepends a TAB character to each
> line).
> 
>>  +(defsubst diff-goto-line-before-patch-separator ()
>>  +  "Go to prev. line, then if it has patch separator as produced
>>  +by git-format-patch, stay there. Otherwise go back."
> 
> The first line of a doc string should be a complete sentence.  I
> suggest to rephrase as follows:
> 
>   Return buffer position before patch separator produced by 
> git-format-patch.

Thank you, I'll address the comments a bit later this day!

>>  +  (previous-line)
>>  +  (when (not (looking-at "-- "))
>>  +      (next-line))
>>  +  (point))
> 
> Btw, Diff mode is more general than just Git-produced diffs.  Is there
> any possibility that this change will misfire in diffs produced by
> other tools?  If so, perhaps we should also verify the buffer is under
> Git control.

Oh, while writing this, I figured I should change the regexp from `-- ` 
to `^-- $`. Will do.

With that addressed: very unlikely. A miscalculation could happen if 
all of the following holds true *simultaneously*:

	* The diff removes a line with the exact text `- `, i.e. two 
characters {dash,space}. That would result in {dash,dash,space} diff, 
same as patch separator.
	* The line was the last line removed in the hunk.
	* The hunk has no context after the removed line

Note, these need to hold true simultaneously. Give the low probability 
of this compared to git-format-patch that is used literally everywhere 
(and that without this patch diff-mode can't handle it), I think this 
is a reasonable trade-off.