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