bug#38677: 26.3; `display-buffer-reuse-mode-window' doesn't check for derived modes

[Trevor Murphy]

The problem is in the middle of the function:

        (let ((mode?
               (with-current-buffer (window-buffer window)
                 (cond ((memq major-mode allowed-modes)
                        'same)
                       ((derived-mode-p allowed-modes) ; <= this line
                        'derived)))))
                        ...)

`allowed-modes' is always a list at this point, but `derived-mode-p'
expects each mode as a separated argument (it takes them with an &rest
parameter).  So this call always fails.