lib: add private function to extract the database for a message.

This is needed by logging in functions outside message.cc that take
only a notmuch_message_t object.

lib/message.cc

@@ -1626,3 +1626,9 @@ notmuch_message_destroy (notmuch_message_t *message)
 {
     talloc_free (message);
 }
+
+notmuch_database_t *
+_notmuch_message_database (notmuch_message_t *message)
+{
+    return message->notmuch;
+}

lib/notmuch-private.h

@@ -472,6 +472,8 @@ _notmuch_doc_id_set_remove (notmuch_doc_id_set_t *doc_ids,
 void
 _notmuch_message_add_reply (notmuch_message_t *message,
 			    notmuch_message_t *reply);
+notmuch_database_t *
+_notmuch_message_database (notmuch_message_t *message);
 
 /* sha1.c */