repair._bundle: fix traceback for bad config value
On IRC, rom1dep reported a traceback[1] from setting
experimental.strip-bundle2-version to True. This diff catches
unexpected values and falls back to the non-experimental bundle1
implementation after issuing a warning.
[1] http://gist.tamytro.org/_admin/gists/qXcdQLwtApgy6e3NwWgl
Run Mercurial tests with Vagrant:
$ vagrant up
$ vagrant ssh -c ./run-tests.sh