hl-todo--combine-face: Use list of faces instead of inheriting

Prepend ((:foreground ...) hl-todo) faces instead of (:inherit hl-todo
:foreground ...).  Using the hl-todo face directly makes it a little
easier to inspect the face property and to integrate with other
packages like Embark.

hl-todo.el

@@ -254,9 +254,9 @@ including alphanumeric characters, cannot be used here."
 
 (defun hl-todo--combine-face (face)
   (if (stringp face)
-      (list :inherit 'hl-todo
-            (if hl-todo-color-background :background :foreground)
-            face)
+      `((,(if hl-todo-color-background :background :foreground)
+         ,face)
+        hl-todo)
     face))
 
 (defvar-keymap hl-todo-mode-map