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.
http://example.com/no/anchor, branches: (None, [])
http://example.com/an/anchor, branches: ('foo', [])
http://example.com/no/anchor/branches, branches: (None, ['foo'])
http://example.com/an/anchor/branches, branches: ('bar', ['foo'])
http://example.com/an/anchor/branches-None, branches: ('foo', [])
http://example.com/, branches: (None, [])
http://example.com/, branches: (None, [])
http://example.com/, branches: ('foo', [])