Undo information in nnfolder buffers.

[Lute Kamstra]

I use nnfolder as my mail back end.  Being the control-freak that I
am, I don't expire nothing, ever:

(setq gnus-secondary-select-methods
      '((nnfolder "mail"
                  (nnfolder-get-new-mail t)
                  (nnfolder-inhibit-expiry t))))

As a result, my spam group tends to grow rather big.  So once in a
while I pay it a visit and do M P b and B <DEL> to send the spam to
/dev/null.  With CVS Emacs, this gives me loads of messages like this:

  Buffer spam undo info is 3234240 bytes long; discard it? (yes or no)

Is it necessary to record undo information in nnfolder buffers?  Or in
other back ends for that matter?  Human editing in back ends is
explicitly requested and terminated, so it should be easy to enable
and disable recording undo information at those moments.

Lute.