add basic http proxy tests

add basic http proxy tests

We do not test our http proxy functionality at all in the test suite, so
this is a pretty big blind spot. Let's at least add a basic check that
we can go through an authenticating proxy to perform a clone.

A few notes on the implementation:

  - I'm using a single apache instance to proxy to itself. This seems to
    work fine in practice, and we can check with a test that this rather
    unusual setup is doing what we expect.

  - I've put the proxy tests into their own script, and it's the only
    one which loads the apache proxy config. If any platform can't
    handle this (e.g., doesn't have the right modules), the start_httpd
    step should fail and gracefully skip the rest of the script (but all
    the other http tests in existing scripts will continue to run).

  - I used a separate passwd file to make sure we don't ever get
    confused between proxy and regular auth credentials. It's using the
    antiquated crypt() format. This is a terrible choice security-wise
    in the modern age, but it's what our existing passwd file uses, and
    should be portable. It would probably be reasonable to switch both
    of these to bcrypt, but we can do that in a separate patch.

  - On the client side, we test two situations with credentials: when
    they are present in the url, and when the username is present but we
    prompt for the password. I think we should be able to handle the
    case that _neither_ is present, but an HTTP 407 causes us to prompt
    for them. However, this doesn't seem to work. That's either a bug,
    or at the very least an opportunity for a feature, but I punted on
    it for now. The point of this patch is just getting basic coverage,
    and we can explore possible deficiencies later.

  - this doesn't work with LIB_HTTPD_SSL. This probably would be
    valuable to have, as https over an http proxy is totally different
    (it uses CONNECT to tunnel the session). But adding in
    mod_proxy_connect and some basic config didn't seem to work for me,
    so I punted for now. Much of the rest of the test suite does not
    currently work with LIB_HTTPD_SSL either, so we shouldn't be making
    anything much worse here.

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