histedit: switch state to store node instead of ctx Currently, if the node no longer exists, the state object fails to load and pukes with an exception. Changing the state object to only store the node allows callers to handle these cases. For instance, in bootstrapcontinue we can now detect that the node doesn't exist and exit gracefully. The alternative is to have the state object store something like None when the node doesn't exist, but then outside callers won't be able to access the old node for recovery (unless we store both the node and the ctx, but why bother). More importantly it allows us to detect this case when doing hg histedit --abort. Currently this situation results in both --continue and --abort being broken and the user has to rm .hg/histedit-state to unwedge their repo. (description by Durham Goode)