D4131: dirstate: use visitchildrenset in traverse

[Yuya Nishihara]

Queued the series, thanks.

>              wadd = work.append
>              while work:
>                  nd = work.pop()
> -                if not match.visitdir(nd):
> +                visitentries = match.visitchildrenset(nd)
> +                if not visitentries:
>                      continue
> +                if visitentries == 'this' or visitentries == 'all':
> +                    visitentries = None
>                  skip = None
>                  if nd == '.':
>                      nd = ''
> @@ -909,6 +912,8 @@
>                          continue
>                      raise
>                  for f, kind, st in entries:
> +                    if visitentries and f not in visitentries:
> +                        continue

Correct me if I get it wrong:

- visitentries is a list of direct sub-directories.
- if there are any files to visit, visitentries should be either 'this' or 'all'.
- so we don't have to check if `f` is a directory or not.