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Thanks, this is a great PR! It has one big problem which is that it adds a new GitAnchorType BRANCH. I think that's a bad idea, but I'm happy to merge it anyway.

But! We should merge Bitbucket support without BRANCH support. Then, after that has been merged, I'd be happy to take a new PR which adds BRANCH support. I just created a new issue for mutable branch support in #24.

If you delete support for BRANCH, it will be quick and easy to get this merged. From there, it's also easy to close out #24 if you still want to.