Allow programs to not depend on remotes having urls

Allow programs to not depend on remotes having urls

For fetch and ls-remote, which use the first url of a remote, have
transport_get() determine this by passing a remote and passing NULL
for the url. For push, which uses every url of a remote, use each url
in turn if there are any, and use NULL if there are none.

This will allow the transport code to do something different if the
location is not specified with a url.

Also, have the message for a fetch say "foreign" if there is no url.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>