http: always use any proxy auth method available

http: always use any proxy auth method available

We set CURLOPT_PROXYAUTH to use the most secure authentication
method available only when the user has set configuration variables
to specify a proxy.  However, libcurl also supports specifying a
proxy through environment variables.  In that case libcurl defaults
to only using the Basic proxy authentication method, because we do
not use CURLOPT_PROXYAUTH.

Set CURLOPT_PROXYAUTH to always use the most secure authentication
method available, even when there is no git configuration telling us
to use a proxy. This allows the user to use environment variables to
configure a proxy that requires an authentication method different
from Basic.

Signed-off-by: Enrique A. Tobis <etobis@twosigma.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>