transplant: add checkunfinished (issue3955) Nobody loves transplant anymore, but it was the first command to have a --continue flag.

histedit: add checkunfinished support (issue3955) The tests contain a couple corner cases where workarounds are now required. Strictly speaking, these are behavior regressions, but of an extremely obscure and marginal sort. Commits or updates in the middle of a histedit would have almost always been fairly serious user error.