WSGI: avoid leaking repository class objects
This is the same as hgitaly!59 (merged), except that we can be confident
that we aren't serving a repo that was created by hg share.
Mutualization of some kind would obviously be beneficial, but is prematurate, depending on what we can do upstream (Mercurial or topic extension).
See also heptapod#466 (closed) where the impact on HTTP server is mentioned