Feature request: support cascading merge (similar to the cascading rebase)

[hedgesky]

In our company, we loved the experience of the Github Stacks!

However, it seems that we won't be able to rely on them fully, as they only support cascading rebases, and our branching policy forbids using them.

Instead, we would love having possibility to perform "cascading merges", as if we'd click the "Update branch" button for each PR in the stack.