add_message: Re-order the code a bit (find message-id first).

add_message: Re-order the code a bit (find message-id first).

We're preparing for being able to deal with files with duplicate
message IDs here. The plan is to create a notmuch_message_t object in
add_message that may or may not reference a document that exists in
the database. So to do this, we have to find the message ID before we
do any manipulation of the doc.