use distinct username/password for http auth tests

The httpd server we set up to test git's http client code
knows about a single account, in which both the username and
password are "user@host" (the unusual use of the "@" here is
to verify that we handle the character correctly when URL
escaped).

This means that we may miss a certain class of errors in
which the username and password are mixed up internally by
git. We can make our tests more robust by having distinct
values for the username and password.

In addition to tweaking the server passwd file and the
client URL, we must teach the "askpass" harness to accept
multiple values. As a bonus, this makes the setup of some
tests more obvious; when we are expecting git to ask
only about the password, we can seed the username askpass
response with a bogus value.

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

t/lib-httpd/passwd

@@ -1 +1 @@
-user@host:nKpa8pZUHx/ic
+user@host:xb4E8pqD81KQs