Syntactic fontification of diff hunks

[Juri Linkov <juri@linkov.net>]

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Most version control sites like gitlab/github highlight syntax
in code snippets inside diff hunks, for example:
https://github.com/magit/magit/pull/2834/commits/95cacde4fcccc95c25d6fb9988d2aa097193f8c0

This is very helpful when looking at code changes.  I missed this feature in Emacs
for a long time.  This is why I asked a question about a possible implementation in
https://emacs.stackexchange.com/questions/43957/syntactic-fontification-of-diff-hunks
but no one had an answer.

Then I realized that much simpler would be just to use the same approach
implemented by diff-mode refinement, i.e. to take each diff hunk one by
one, and like the diff refinement highlights more fine-grained changes,
do the same for syntax highlighting according to the language in
compared files/commits.

You can see the result at the following screenshot:


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