subrepo: add "_cachestorehashvfs" to handle cache store hash files via vfs

This "vfs" object will be used by subsequent patches to handle cache
store hash files without direct file APIs.

This patch decorates "_cachestorehashvfs" with "@propertycache" to
delay vfs creation, because it is used only for cooperation with other
repositories.

In this patch, "/" is used as the path separator, even though
"self._repo.join" 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

@@ -566,6 +566,10 b' class hgsubrepo(abstractsubrepo):'
         return self._repo.join(os.path.join(
             'cache', 'storehash', _getstorehashcachename(remotepath)))
 
+    @propertycache
+    def _cachestorehashvfs(self):
+        return scmutil.vfs(self._repo.join('cache/storehash'))
+
     def _readstorehashcache(self, remotepath):
         '''read the store hash cache for a given remote repository'''
         cachefile = self._getstorehashcachepath(remotepath)