hgweb: Respond with HTTP 403 for disabled archive types instead of 404
This makes it easier for clients/users to distinct between supported
but disabled and unsupported archive types.
make test-mq-qdelete work reliably (
issue1180)
if after a and b has been applied, their hash starts with c, then the test
would fail (because there would be a rev c which would be seen as "applied")
so probability of failure would be 2*1/16.