M-- comment-region doesn't remove comment-padding when no more comment chars

[Drew Adams]

In Emacs 20, M-- comment-region effectively undid the operation of
comment-region, if given the right number as prefix arg. In Emacs 21, this
is no longer the case.

In Emacs 21, the comment chars are removed by M--, but the comment-padding
is never removed. This requires users to perform an extra step of M-- C-x
TAB to remove the extra padding inserted by comment-region.

Example:

C-u 5 comment-region on these lines in Emacs-Lisp mode:

xxxxxxxx
yyyyyyyy
zzzzzzzz

Result:

;;;;; xxxxxxxx
;;;;; yyyyyyyy
;;;;; zzzzzzzz

M-- 5 comment-region results in this:

 xxxxxxxx
 yyyyyyyy
 zzzzzzzz

It should result in the original text:

xxxxxxxx
yyyyyyyy
zzzzzzzz