subrepo: add wvfs field to access the working directory via vfs This patch doesn't create vfs object in "abstractsubrepo.__init__()" but adds propertycache-ed "wvfs" field, because the latter can: - delay vfs instantiation until it is actually needed - allow to use "hgsubrepo._repo.wvfs" as "wvfs" "hgsubrepo._repo" is initialized after "abstractsubrepo.__init__()" invocation, and passing "hgsubrepo._repo.wvfs" to "abstractsubrepo.__init__()" is difficult.