D3968: amend: support "history-editing-backup" config option

[Yuya Nishihara]

> @yuja Is there any command left which we can consider to add
> 'history-editing-backup' config option?

Maybe some of these?

```
% rg scmutil.cleanupnodes hgext mercurial
hgext/rebase.py:1760:    scmutil.cleanupnodes(repo, replacements, 'rebase', moves, backup=backup)
hgext/split.py:172:    scmutil.cleanupnodes(repo, {ctx.node(): [c.node() for c in committed]},
hgext/uncommit.py:181:            scmutil.cleanupnodes(repo, mapping, 'uncommit', fixphase=True)
hgext/uncommit.py:250:        scmutil.cleanupnodes(repo, mapping, 'unamend', fixphase=True)
hgext/histedit.py:1211:    scmutil.cleanupnodes(repo, mapping, 'histedit')
mercurial/cmdutil.py:798:        scmutil.cleanupnodes(repo, replacements, 'branch-change', fixphase=True)
mercurial/cmdutil.py:2557:        scmutil.cleanupnodes(repo, mapping, 'amend', metadata=obsmetadata,
hgext/fix.py:188:    """Calls scmutil.cleanupnodes() with the given replacements.
hgext/fix.py:201:    scmutil.cleanupnodes(repo, replacements, 'fix', fixphase=True)
```