tests/test-mq-qsave.t

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.

  $ echo "[extensions]" >> $HGRCPATH
  $ echo "mq=" >> $HGRCPATH

  $ hg init

  $ echo 'base' > base
  $ hg ci -Ambase
  adding base

  $ hg qnew -mmqbase mqbase

  $ hg qsave
  $ hg qrestore 2
  restoring status: hg patches saved state