Syntactic fontification of diff hunks (bug#33567)

* lisp/vc/diff-mode.el (diff-font-lock-syntax): New defcustom.
(diff-default-directory): New buffer-local variable.
(diff-indicator-removed, diff-indicator-added)
(diff-indicator-changed): Set foreground to distinctive colors.
(diff-context): Remove colors to make room for syntax highlighting.
(diff-font-lock-keywords): Add diff--font-lock-syntax.
(diff--font-lock-cleanup): Remove diff-mode syntax overlays.
(diff--font-lock-syntax, diff--font-lock-syntax--refresh)
(diff-syntax-fontify-revisions, diff-syntax-fontify-hunk)
(diff-syntax-fontify-props): New functions.

* lisp/vc/diff.el (diff-no-select): Set diff-default-directory to
default-directory.

* doc/emacs/files.texi (Diff Mode): Document diff-font-lock-syntax.

1 files changed, 6 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 95647bbda4f..bc76bec2d75 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -413,6 +413,12 @@ and compares their entire trees.
 *** Hunks are now automatically refined by default.
 To disable it, set the new defcustom 'diff-font-lock-refine' to nil.
 
++++
+*** 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.
+
 *** File headers can be shortened, mimicking Magit's diff format.
 To enable it, set the new defcustom 'diff-font-lock-prettify' to t.