v3.4-beta1 multiple parallel branches

[mkurjanski]

@jeremystretch
I've been reviewing the extras.models.Branch code added in 3.4 beta and this feature looks very promising and useful.
In the current shape it looks however like the branch is only active within the 'with/checkout' block. This only allows to check out a single branch at a time.

I'm wondering if it would be possible to checkout/stage two or more branches at the same time. Let's say I use the get_next_available_ip() logic in BranchX. If I then create BranchY and call get_next_available_ip() it should be aware the IP was already allocated in another branch.

This would allow to stage multiple parallel changes and commit or roll back only selected branches.

Unless multiple checkouts are going to be supported?
Example:
with checkout(branch1), checkout(branch2):
pass