Overview

This PR makes upgrade work like update when it fails – the branch it puts you on has all dependents of the failures removed from the underlying namespace, so deleting any from the scratch file functions like a delete.

It also makes a successful update on an "upgrade branch" merge the branch back into the parent, like a successful update on an "update branch". So, upgrade.commit is no longer necessary and was replaced with a stub implementation.

Finally, I elected to make upgrade on both an "update branch" and "upgrade branch" fail immediately with an error message. I think we can revisit this and try to find a sensible implementation, but you basically don't ever want to do this with the current implementation, so I think it's better to disallow it. The workaround is to upgrade one library at a time (if trying to upgrade while resolving a failed upgrade), or upgrade before updating (if trying to upgrade while resolving a failed update).

Test coverage

I added a case to the existing transcript to demonstrate that upgrade has this delete-y behavior, and also manually tested in the terminal.