send-pack: fix push.negotiate with remote helper

send-pack: fix push.negotiate with remote helper

Commit 477673d6f3 ("send-pack: support push negotiation", 2021-05-05)
introduced the push.negotiate config variable and included a test. The
test only covered pushing without a remote helper, so the fact that
pushing with a remote helper doesn't work went unnoticed.

This is ultimately caused by the "url" field not being set in the args
struct. This field being unset probably went unnoticed because besides
push negotiation, this field is only used to generate a "pushee" line in
a push cert (and if not given, no such line is generated). Therefore,
set this field.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>