Drop face from hi-lock--unused-faces only when used

* lisp/hi-lock.el (hi-lock-set-pattern): If REGEXP is already
highlighted, then push FACE into hi-lock--unused-faces (Bug#26666).
* test/lisp/hi-lock-tests.el (hi-lock-bug26666): Add test.

lisp/hi-lock.el

@@ -695,7 +695,8 @@ with completion and history."
   (setq regexp (hi-lock--hashcons regexp))
   (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)
+    (if (assoc regexp hi-lock-interactive-patterns)
+        (add-to-list 'hi-lock--unused-faces (face-name face))
       (push pattern hi-lock-interactive-patterns)
       (if (and font-lock-mode (font-lock-specified-p major-mode))
 	  (progn

test/lisp/hi-lock-tests.el

@@ -0,0 +1,40 @@
+;;; hi-lock-tests.el --- Tests for hi-lock.el  -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2017 Free Software Foundation, Inc.
+
+;; Author: Tino Calancha <tino.calancha@gmail.com>
+;; Keywords:
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+;;; Code:
+
+(require 'ert)
+(require 'hi-lock)
+
+(ert-deftest hi-lock-bug26666 ()
+  "Test for http://debbugs.gnu.org/26666 ."
+  (let ((faces hi-lock-face-defaults))
+    (with-temp-buffer
+      (insert "a A b B\n")
+      (cl-letf (((symbol-function 'completing-read)
+                   (lambda (prompt coll x y z hist defaults)
+                     (car defaults))))
+        (dotimes (_ 2)
+          (let ((face (hi-lock-read-face-name)))
+            (hi-lock-set-pattern "a" face))))
+      (should (equal hi-lock--unused-faces (cdr faces))))))
+
+(provide 'hi-lock-tests)
+;;; hi-lock-tests.el ends here