notmuch: fix getting count of new messages for non-trivial query

Currtly, we ask for count of new messages by appending tag:unread (or
whatever NotmuchUnreadTag has) to db query. It works if your query is
very simple: no 'OR', 'NOT', etc.

If you will ask new messages for query like
 "to:foo@example.com or to:bar@example.net"
mutt will query db for count of
 "to:foo@example.com or to:bar@example.net tag:unread"
and notmuch will understand it as
 "to:foo@example.com or (to:bar@example.net tag:unread)"
which, obviously, is not what you wanted.

Optimizations like
	if (strstr(db_query, NotmuchUnreadTag))
		*new = all ? *all : count_query(db, db_query);
doesn't work for the same reason.

Let's fix all this by construct query with proper parenthesis:
 "(to:foo@example.com or to:bar@example.net) tag:unread"

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>