Stack Closes Intermediate PR

[broguinn]

I had a very frustrating experience with stacks today. Here's the first PR in the stack: https://github.com/Semi-Stow/api/pull/3435

This stack was originally five PRs. This afternoon, I added the first->third PRs to the merge queue. When the second PR passed the merge queue validation and merged, the third PR was automatically closed, and the branch was deleted - which ejected it from the merge queue.

According to the github API, that PR was closed due to a manual request from me, though I doubt this is what happened.

This isn't where the frustration ended, though: I was unable to re-open the PR, because the branch was deleted. I also was unable to delete the stack and create a new one, because some parts of the stack had already merged. Or, at least, that's what the UI said - but it appears that clicking that button ejected the third through fifth PRs from the stack.

All of this was very confusing. When stacks work, they're excellent. But when they break, it seems almost impossible to untangle them.