D4477: merge: move purge logic from extension

[Yuya Nishihara]

Queued the series, thanks.

> +    matcher = scmutil.match(repo[None], dirs or (), {
> +        'include': opts.get('include'),
> +        'exclude': opts.get('exclude')})
>  
> -    match = scmutil.match(repo[None], dirs, opts)

Restored the original `match` construction, which looks simpler.