http: add option to try authentication without username
Performing GSS-Negotiate authentication using Kerberos does not require
specifying a username or password, since that information is already
included in the ticket itself. However, libcurl refuses to perform
authentication if it has not been provided with a username and password.
Add an option, http.emptyAuth, that provides libcurl with an empty
username and password to make it attempt authentication anyway.
Signed-off-by: Junio C Hamano <gitster@pobox.com>