[merge] [#252428] allow upgrade of stacked branches

[Bundle Buggy]

Bundle Buggy has detected this merge request.

For details, see: 
http://bundlebuggy.aaronbentley.com/project/bzr/request/%3Ce01316480808040104q1434ba8esb1ef17344116dbf1%40mail.gmail.com%3E
Project: Bazaar