bug#72556: 29.1.90; vc-diff does not undo hunk in end of source file

[Juri Linkov <juri@linkov.net>]

>> To reproduce:
>>
>> $ mkdir apply
>> $ cd apply
>> $ echo "first line of code" > example.file
>> $ git init
>> $ git add .
>> $ git commit -m 'init'
>> $ echo "last line of code" >> example.file
>> $ emacs -Q example.file
>>
>> C-x v =
>> n
>> C-c C-a
>>
>> The hunk is applied in the source buffer.
>
> Juri and Dmitry, any comments or suggestions?

I don't know why it fails at the end, so I tried to debug 'diff-apply-hunk',
and found that the problem is in these lines of 'diff-find-source-location':

    ;; FIXME: Check for case where both OLD and NEW are found.
    (pos (or (diff-find-text (car old))

When the hunk is at the beginning/end of the diff buffer,
then it misses the top/bottom part of the context,
so 'diff-find-text' wrongly matches it because it fails
to see any changes in it due to insufficient context,
and returns switched=nil.

A possible solution would be to add a condition to detect the case
when the hunk is at the beginning/end of the diff buffer.