bug#33567: Syntactic fontification of diff hunks

[Eli Zaretskii <eliz@gnu.org>]

> From: Juri Linkov <juri@linkov.net>
> Cc: 33567@debbugs.gnu.org
> Date: Thu, 13 Dec 2018 01:17:52 +0200
> 
> Here is a more less final version that I tested on many different diffs:

Thanks, I have a few minor comments, mainly to the documentation
parts.

> diff --git a/etc/NEWS b/etc/NEWS
> index decc5e3954..1024e9e9f5 100644
> --- a/etc/NEWS
> +++ b/etc/NEWS
> @@ -273,6 +273,8 @@ To disable it, set the new defcustom 'diff-font-lock-refine' to nil.
>  *** File headers can be shortened, mimicking Magit's diff format
>  To enable it, set the new defcustom 'diff-font-lock-prettify to t.
>  
> +*** Source language syntax is highlighted in diff hunks when 'diff-font-lock-syntax' is t.

This line is too long, suggest to rephrase:

  *** Better syntax highlighting of Diff hunks.
  Fragments of source in Diff hunks are now by default highlighted
  according to the appropriate major mode.  Customize the new option
  'diff-font-lock-syntax' to nil to disable this.

> +@vindex diff-font-lock-syntax
> +  If non-nil, diff hunk font-lock includes source language syntax highlighting.

"nil" should be in @code.

> diff --git a/lisp/vc/diff.el b/lisp/vc/diff.el
> index ac94586cac..93fa54b1fb 100644
> --- a/lisp/vc/diff.el
> +++ b/lisp/vc/diff.el
> @@ -165,6 +167,7 @@ diff-no-select
>             (lambda (_ignore-auto _noconfirm)
>               (diff-no-select old new switches no-async (current-buffer))))
>        (setq default-directory thisdir)
> +      (set (make-local-variable 'diff-default-directory) default-directory)

Any reason not to use setq-local?

> +(defun diff--font-lock-syntax (max)
> +  "Syntax highlighting from font-lock."

Although an internal function, could it have a slightly more detailed
do string, please?

> +(defun diff-syntax-fontify-hunk (beg end old)
> +  "Highlight source language syntax in diff hunks."

This is for a single hunk, not "hunks", right?  Also, please mention
the arguments in the doc string.

> +(defun diff-syntax-fontify-props (file text line-nb &optional no-init hunk-only)
> +  "Get font-lock properties from the source code."

Please mention the arguments ion the doc string.