transport-helper: update remote helper namespace

When pushing, the remote namespace is updated correctly
(e.g. refs/origin/master), but not the remote helper's
(e.g. refs/testgit/origin/master).

Let's update it to point at the version we pushed out, so that the
next run of the helper knows where to start.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>