[MERGE] Implement and test Tree.get_file_sha1 and Tree.__contains__ for previewtree

[Ian Clatworthy]

Ian Clatworthy has voted approve.
Status is now: Approved


For details, see: 
http://bundlebuggy.aaronbentley.com/project/bzr/request/%3C48D87904.9030904%40aaronbentley.com%3E
Project: Bazaar