bug#14645: Keep highlighting face foreground

[Juri Linkov]

I was going to propose a similar change to use `add-face-text-property'
in hi-lock, but it turned out to be much simpler because hi-lock
is based on font-lock that already has `font-lock-prepend-text-property'
that does the same:

=== modified file 'lisp/hi-lock.el'
--- lisp/hi-lock.el     2013-06-03 08:51:50 +0000
+++ lisp/hi-lock.el     2013-06-19 23:12:05 +0000
@@ -715,7 +690,7 @@ (defun hi-lock-set-pattern (regexp face)
   "Highlight REGEXP with face FACE."
   ;; Hashcons the regexp, so it can be passed to remove-overlays later.
   (setq regexp (hi-lock--hashcons regexp))
-  (let ((pattern (list regexp (list 0 (list 'quote face) t))))
+  (let ((pattern (list regexp (list 0 (list 'quote face) 'prepend))))
     ;; Refuse to highlight a text that is already highlighted.
     (unless (assoc regexp hi-lock-interactive-patterns)
       (push pattern hi-lock-interactive-patterns)