subrepo: replace "os.path.exists" by "exists" via wvfs of the parent

Existance of specified "path" should be examined by "exists" via wvfs
of the parent repository, because the working directory of the parent
repository may be in UTF-8 mode. Wide API should be used via wvfs in
such case.

In this patch, "/" is used as the path separator, even though "path"
uses platform specific path separator (e.g. "\\" on Windows). But it
is reasonable enough, because "store" and other management file
handling already include such implementation, and they work well.

mercurial/subrepo.py

@@ -522,7 +522,7 b' class hgsubrepo(abstractsubrepo):'
         self._state = state
         r = ctx._repo
         root = r.wjoin(path)
-        create = not os.path.exists(os.path.join(root, '.hg'))
+        create = not r.wvfs.exists('%s/.hg' % path)
         self._repo = hg.repository(r.baseui, root, create=create)
         for s, k in [('ui', 'commitsubrepos')]:
             v = r.ui.config(s, k)