bzr push fails if there is no "master" bzr branch

[termie]

this is the result of a relatively quick fix to make sure that we are preserving bzr history so bzr users can merge the branches we create, unfortunately if we don't have a bzr branch named 'master' (one is made if we did a git bzr clone, but not if we did a git bzr import without specifying master as the branch name, which is unlikely to happen).

A workaround if you run into such a position is to make a new git repo via bzr clone and then just pull the changes in your other repo into it, the ids should be the same.

[termie]

testing mentions for organizations @cloudbuilders