Feature Request: Support DAG's

[d4hines]

Hi,
Suppose I'm working on big feature A. Along the way I make small improvements B, C, and D. I can structure a stack like:

A -> B -> C -> D-> main

But suppose B, C, D are all orthogonal to each other. The above structure imposes an order that isn't necessary and may not even make sense (the diff on github will compare C with D, but we'd like to compare C with main. If D is controversial, review may take a long time, but B and C might be able to be quickly reviewed and merged, minimizing the distance of A to main (which is the end goal).

What I would like is a DAG:

              A
          /   |   \
        B     C     D
         \    |    /
            main

This would allow us to review and merge with maximal "concurrency".
I'd like gh-stack to keep track of this dependency graph and automatically do the required rebasing on changes.

Is this possible? None of the existing stacked git tools seem to support this so I'm wondering if there is some fatal flaw.