cli/reply: pull proposed subject line from the message, not the index

cli/reply: pull proposed subject line from the message, not the index

Protected subject lines were being emitted in reply when the cleartext
of documents was indexed.  create_reply_message() was pulling the
subject line from the index, rather than pulling it from the
GMimeMessage object that it already has on hand.

This one-line fix to notmuch-reply.c solves that problem, and doesn't
cause any additional tests to fail.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>