Update: I have succeeded in recovering from this, but I still have one question and one feature request:
1. Is it possible to use the graphite UI to merge stacks if commits are required to be signed? Are there instructions for handling that somewhere?
2. As far as I can tell from my experience here, if I use the merge button in the graphite UI to merge a stack, and it fails partway through, my whole stack locally is going to be messed up. It would be better if that were not the case.