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, 3 insertions, 0 deletions

diff --git a/lisp/vc/diff.el b/lisp/vc/diff.el
index ac94586cace..ed5b49d3bf4 100644
--- a/lisp/vc/diff.el
+++ b/lisp/vc/diff.el
@@ -121,6 +121,8 @@ Possible values are:
   nil   -- no, it does not
   check -- try to probe whether it does")
 
+(defvar diff-default-directory)
+
 (defun diff-no-select (old new &optional switches no-async buf)
   ;; Noninteractive helper for creating and reverting diff buffers
   (unless (bufferp new) (setq new (expand-file-name new)))
@@ -165,6 +167,7 @@ Possible values are:
            (lambda (_ignore-auto _noconfirm)
              (diff-no-select old new switches no-async (current-buffer))))
       (setq default-directory thisdir)
+      (setq diff-default-directory default-directory)
       (let ((inhibit-read-only t))
         (insert command "\n"))
       (if (and (not no-async) (fboundp 'make-process))