`lastBuild` is a synchronous relationship on a branch model, so we need to
have a build record present when we put a default branch from a repository
model into the store. We don't send lastBuild's payload in pusher, so
we need to get it using an ajax call, if it's not already in the store.
In the future we may decide to make the relationship async, but I don't
want to change the code at the moment
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