Option to prefer to base branch for new branches

[peterhoeg]

Currently, sb will always create new branches based on the current checkout.

It would be great to have an option to specify a base branch (such as origin/develop) per project which could then be used to create new branches from and a boolean flag on the project to always use the base branch or having to specify it. This would support the following use cases:

1. no default upstream, no use upstream: always create new branch from current checkout
2. default upstream, use upstream set to false: create new branch from current checkout but allow using default upstream if passed a parameter (--upstream)
3. default upstream, use upstream set to true: create new branch from upstream but allow using current checkout if passed a parameter (--current)

Additional information

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