push options: pass push options to the transport helper

push options: pass push options to the transport helper

When using non-builtin protocols relying on a transport helper
(such as http), push options are not propagated to the helper.

The user could ask for push options and a push would seemingly succeed,
but the push options would never be transported to the server,
misleading the users expectation.

Fix this by propagating the push options to the transport helper.

This is only addressing the first issue of
   (1) the helper protocol does not propagate push-option
   (2) the http helper is not prepared to handle push-option

Once we fix (2), the http transport helper can make use of push options
as well, but that happens as a follow up. (1) is a bug fix, whereas (2)
is a feature, which is why we only do (1) here.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>