t: test http access to "half-auth" repositories

t: test http access to "half-auth" repositories

Some sites set up http access to repositories such that
fetching is anonymous and unauthenticated, but pushing is
authenticated. While there are multiple ways to do this, the
technique advertised in the git-http-backend manpage is to
block access to locations matching "/git-receive-pack$".

Let's emulate that advice in our test setup, which makes it
clear that this advice does not actually work.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>