[MERGE] Make Revision.get_summary() return an empty string, instead of backtracing

[Bundle Buggy]

Bundle Buggy has detected this merge request.

For details, see: 
http://bundlebuggy.aaronbentley.com/project/bzr/request/%3Ca1c1f4750906030326w28b68141wca57061b532aa3fe%40mail.gmail.com%3E
Project: Bazaar