D3887: rebase: add --no-backup option

[Yuya Nishihara]

>   @yuja @pulkit Definitely, adding a config option is better than any other alternative. Can I start to work on this? (I mean adding config option which will work for all history editing commands)

Sounds good to me, though I don't have any good name for the config option.

To be clear, I think `hg strip` should be out of the control of the config
option to be added, because the strip in that case isn't a side effect of
history editing.