emacs: tree bugfix

Formerly replying to an encrypted message in tree-view did not work:
the message was not decrypted. This commit makes notmuch-tree respect
the setting of notmuch-crypto-process-mime. In particular, if
notmuch-crypto-process-mime is set to t, then replying to encrypted
messages in tree mode will now decrypt the reply (as it already did in
show mode).