Update only the top PR in stack

[erwang01]

Is it possible to push updates to a specific PR in a stack?

This is particularly useful when iterating over a stack and we only want to run CI on a specific PR in the stack. I know we skip updates if there has been no changes but when rebasing against the main / trunk branch, all commits in the stack are considered changed even if you only needed the update in specific commits on your branch.

[ezyang]

This would indeed be a useful feature, but it requires some surgery to the code implementation that i haven't gotten around to doing

[ezyang]

Duping as #101