localrepo: use the vfs join method to implement the localrepo join method
This will make it possible to customize the behavior of the join method by
changing the vfs class (e.g. by using the altvfs" class introduced recently).
Note that we could have modified the VFS join methods to acept a set of optional
paths in the same way thta the localrepo join method does. However it seemed
simpler to simply call os.path.join before calling self.vfs.join.