t5540: test DAV push with authentication

t5540: test DAV push with authentication

We don't currently test this case at all, and instead just
test the DAV mechanism over an unauthenticated push. That
isn't very realistic, as most people will want to
authenticate pushes.

Two of the tests expect_failure as they reveal bugs:

  1. Pushing without a username in the URL fails to ask for
     credentials when we get an HTTP 401. This has always
     been the case, but it would be nice if it worked like
     smart-http.

  2. Pushing with a username fails to ask for the password
     since 986bbc0 (http: don't always prompt for password,
     2011-11-04). This is a severe regression in v1.7.8, as
     authenticated push-over-DAV is now totally unusable
     unless you have credentials in your .netrc.

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