Allow branches to appear in multiple chains (i.e. support tree-like chains)

[benrucker]

Hey! Love this tool for my workflows. One feature that I wish it had is the ability for a chain to split into different paths when dependencies are not perfectly sequential.

To illustrate, let's say I'm managing a party of 5 branches, A, B, C, D, and E with a main branch called main. A, B, and C depend on each other sequentially

main <- A <- B <- C

This is a perfect chance to use git chain! Now, lets say that D and E depend on changes from C, but not on each other.

main <- A <- B <- C <- D
                  ^--- E

As far as I can tell, there is currently no way to model this in git chain. The workaround is to either not include E as part of any chain until more of the chain merges into main or to add E to the end of the chain despite not needing to rely on D.

I run into this situation fairly often, so I'd love to see support for tree-like chains added!

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Perhaps it would be straightforward to make this work by allowing branches to appear in multiple chains at once? So in the above example, I could create a chain A, B, C, D and a chain C, E, and git chain rebase (and other commands) would check at each branch if there are additional chains to process.