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, 4 insertions, 0 deletions

diff --git a/doc/emacs/files.texi b/doc/emacs/files.texi
index b47be51e24c..6e1faf84dcb 100644
--- a/doc/emacs/files.texi
+++ b/doc/emacs/files.texi
@@ -1617,6 +1617,10 @@ displayed in the echo area).  With a prefix argument, it tries to
 modify the original (``old'') source files rather than the patched
 (``new'') source files.
 
+@vindex diff-font-lock-syntax
+  If non-@code{nil}, fragments of source in hunks are highlighted
+according to the appropriate major mode.
+
 @node Copying and Naming
 @section Copying, Naming and Renaming Files