lib: define specialized get_thread_id for use in thread subquery

The observation is that we are only using the messages to get there
thread_id, which is kindof a pessimal access pattern for the current
notmuch_message_get_thread_id

lib/message.cc

@@ -318,6 +318,23 @@ _notmuch_message_get_term (notmuch_message_t *message,
     return value;
 }
 
+/*
+ * For special applications where we only want the thread id, reading
+ * in all metadata is a heavy I/O penalty.
+ */
+const char *
+_notmuch_message_get_thread_id_only (notmuch_message_t *message)
+{
+
+    Xapian::TermIterator i = message->doc.termlist_begin ();
+    Xapian::TermIterator end = message->doc.termlist_end ();
+
+    message->thread_id = _notmuch_message_get_term (message, i, end,
+						    _find_prefix ("thread"));
+    return message->thread_id;
+}
+
+
 static void
 _notmuch_message_ensure_metadata (notmuch_message_t *message, void *field)
 {

lib/notmuch-private.h

@@ -537,6 +537,10 @@ _notmuch_message_database (notmuch_message_t *message);
 
 void
 _notmuch_message_remove_unprefixed_terms (notmuch_message_t *message);
+
+const char *
+_notmuch_message_get_thread_id_only(notmuch_message_t *message);
+
 /* sha1.c */
 
 char *

lib/thread-fp.cc

@@ -52,7 +52,7 @@ ThreadFieldProcessor::operator() (const std::string & str)
 		std::string term = thread_prefix;
 		notmuch_message_t *message;
 		message = notmuch_messages_get (messages);
-		term += notmuch_message_get_thread_id (message);
+		term += _notmuch_message_get_thread_id_only (message);
 		terms.insert (term);
 	    }
 	    return Xapian::Query (Xapian::Query::OP_OR, terms.begin (), terms.end ());