D3901: histedit: add nobackup config option

[Yuya Nishihara]

> +coreconfigitem('ui', 'historyediting_backup',
> +    default=True,
> +)
>  coreconfigitem('ui', 'interactive',
>      default=None,
>  )
> diff --git a/hgext/histedit.py b/hgext/histedit.py
> --- a/hgext/histedit.py
> +++ b/hgext/histedit.py
> @@ -1111,7 +1111,8 @@
>      fm.startitem()
>      goal = _getgoal(opts)
>      revs = opts.get('rev', [])
> -    nobackup = opts.get('no_backup')
> +    nobackup = (opts.get('no_backup') or
> +                not ui.configbool('ui', 'historyediting_backup'))

`history-editing-backup` per new rule.

https://www.mercurial-scm.org/wiki/UIGuideline#config

Can you add `# experimental config: ui.history-editing-backup` to silence
check-config? It's probably too late to add full support for this option
and make it documented.

And, should we drop the --no-backup option? @pulkit what do you think?