ITS#7722 fix page_merge for collapsing root

http://www.openldap.org/its/index.cgi?findid=7722

commit 940f5cb3736bf21fef1f4faa6c653cf2bb28f78d
Author: Howard Chu <hyc@symas.com>
Date:   Wed Oct 9 02:24:45 2013 -0700

    ITS#7722 fix page_merge for collapsing root

        Other cursors being fixed up are not necessarily height 1.
Merge branch 'master' of github.com:dw/py-lmdb