Combine formatting faces with other highlighting
* lisp/gnus-spec.el (gnus-face-face-function): Initialize the value of
the `face' property with a list whose car is the face specified in the
format string and whose cdr is (nil).
* lisp/gnus-util.el
(gnus-put-text-property-excluding-characters-with-faces): Change
accordingly.
(gnus-get-text-property-excluding-characters-with-faces): New function.
* lisp/gnus-sum.el (gnus-summary-highlight-line):
* lisp/gnus-salt.el (gnus-tree-highlight-node):
* lisp/gnus-group.el (gnus-group-highlight-line): Use it.