D7570: match: resolve filesets against the passed `cwd`, not the current one

[Yuya Nishihara]

>   In D7570#112206 <https://phab.mercurial-scm.org/D7570#112206>, @yuja wrote:
>   Good catch.  I don't see the meaning of b'' in this context defined anywhere.  But since `subinclude:` uses it to define the matcher, I'm assuming that's all of the adjustment we need?

It's basically the value meaning `cwd == repo.root`. See dirstate.getcwd().