Deny invalid path

If an authenticated user calls `hg init hg.host.com/dummy/../../../etc`
it will create the repository in another root directory if the process of
hgkeeper has permissions for this.
This could be an attack to the server.

Also hgkeeper admin repository can be overriden like this.
`hg init ssh://hg.host.com/dummy/../hgkeeper/keys`

Reviewed at https://reviews.imfreedom.org/r/2422/