D4370: treemanifest: use visitchildrenset when filtering a manifest to a matcher

[Yuya Nishihara]

Queued, thanks.

> +    def _loadchildrensetlazy(self, visit):
> +        if not visit:
> +            return None
> +        if visit == 'all' or visit == 'this':
> +            self._loadalllazy()
> +            return None
> +
> +        todel = []
> +        for k in visit:
> +            kslash = k + '/'
> +            ld = self._lazydirs.get(kslash)
> +            if ld:
> +                path, node, readsubtree = ld
> +                self._dirs[kslash] = readsubtree(path, node)
> +                todel.append(kslash)
> +        for kslash in todel:
> +            del self._lazydirs[kslash]

Any reason not to use `self._loadlazy(kslash)`?