bug#50715: 28.0.50; whitespace-report uses fundamental-mode

[Gabriel]

Description:
The command 'whitespace-report' uses 'fundamental-mode' for its buffer
(*Memory Report*). I expect a better default mode that can provide a
better user experience (e.g.: read-only, press 'q' to quit window, etc).
For instance, 'memory-report' uses 'special-mode'.

Steps to reproduce:
1) 'emacs -Q' from master branch
2) M-x 'whitespace-report'

--
        Gabriel