mutt shouldn't write back unchanged mh messages

[Scott Schwartz]

When reading an mh folder mutt 1.0.1 apparently loads every message,
and, when you quit, writes every message back to disk.  (Is that true
for maildir too?) That's surely a bug, right?  The whole point of
one-file-per-message formats is that you only touch the data you need
to, and leave the rest alone.  Otherwise, with a large folder, mutt is
unusably slow, and it also destroys e.g. timestamp information on old
files.