D4876: amend: add config to skip amend if only date is changed (issue5828)

[Yuya Nishihara]

>   I should be easy to introduce such an option and make amend respect it (similar amount of code as this patch), so I'd just like to hear if others think it's a good idea first. If others agree with it, then it seems better to introduce the generic option right away. I can't/won't access IRC right now, but feel free to check there what others think and update this review when you have.

SGTM.

Perhaps, `ui.history-editing-backup` can be moved under the same config group.