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

[Yuya Nishihara]

>   > Any reason not to use `self._loadlazy(kslash)`?
>   
>   Hmm, I feel like there was a previous version that iterated differently and meant I'd need a copy of the list to do this correctly.. in this version, no I don't think there's a reason to avoid it besides a potential minor speed difference; should I send a patch for that?

I don't have any preference, but please send a patch if that makes sense,
thanks.