Syntactic fontification of diff hunks (bug#33567)

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.