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.

mercurial/subrepo.py

@@ -552,6 +552,12 b' class abstractsubrepo(object):'
     def shortid(self, revid):
         return revid
 
+    @propertycache
+    def wvfs(self):
+        """return vfs to access the working directory of this subrepository
+        """
+        return scmutil.vfs(self._ctx.repo().wvfs.join(self._path))
+
 class hgsubrepo(abstractsubrepo):
     def __init__(self, ctx, path, state):
         super(hgsubrepo, self).__init__(ctx, path)
@@ -944,6 +950,12 b' class hgsubrepo(abstractsubrepo):'
     def shortid(self, revid):
         return revid[:12]
 
+    @propertycache
+    def wvfs(self):
+        """return own wvfs for efficiency and consitency
+        """
+        return self._repo.wvfs
+
 class svnsubrepo(abstractsubrepo):
     def __init__(self, ctx, path, state):
         super(svnsubrepo, self).__init__(ctx, path)