emacs/mua: Generate improved cited text for replies

emacs/mua: Generate improved cited text for replies

Use the message display code to generate message text to cite in
replies.

For now we set insert-headers-p function to
notmuch-show-reply-insert-header-p-never so that, as before, we don't
insert part buttons.

With that choice of insert-headers-p function there is only one
failing test: this test has a text part (an email message) listed as
application/octet-stream. Notmuch show displays this part, but the
reply code omitted it as it had type application/octet-stream. The new
code correctly includes it. Thus update the expected output to match.