poor behaviour of lisp-fill-paragraph

[Dave Love]

Consider in Lisp mode this sort of text that I often produce:

  a                                     ; x
  ;; b c
  ;; d
  
M-q on the second or third line produces

  a                                     ; x b c d

not what I think it should:

  a                                     ; x
  ;; b c d

I thought this had been fixed post-21.2, but apparently not.  I can't
remember the logic in my old code which got it right, but I guess it's
just a question of narrowing to a region introduced only by the prefix
around point:  "^\\s-*\\(\\s<+\\)" or similar.