Allow push and fetch urls to be different

Allow push and fetch urls to be different

This introduces a config setting remote.$remotename.pushurl which is
used for pushes only. If absent remote.$remotename.url is used for
pushes and fetches as before.
This is useful, for example, in order to do passwordless fetches
(remote update) over the git transport but pushes over ssh.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>