lib: Explicitly set BoolWeight when searching.

All notmuch searches currently sort by value (either date or message
ID) so it's just wasted effort for Xapian to compute relevance values
for each result. We now explicitly tell Xapian that we're uninterested
in the relevance values.

lib/query.cc

@@ -134,6 +134,8 @@ notmuch_query_search_messages (notmuch_query_t *query)
 					 mail_query, string_query);
 	}
 
+	enquire.set_weighting_scheme (Xapian::BoolWeight());
+
 	switch (query->sort) {
 	case NOTMUCH_SORT_OLDEST_FIRST:
 	    enquire.set_sort_by_value (NOTMUCH_VALUE_TIMESTAMP, FALSE);