Require a struct remote in transport_get()

Require a struct remote in transport_get()

cmd_ls_remote() was calling transport_get() with a NULL remote and a
non-NULL url in the case where it was run outside a git
repository. This involved a bunch of ill-tested special
cases. Instead, simply get the struct remote for the URL with
remote_get(), which works fine outside a git repository, and can also
take global options into account.

This fixes a tiny and obscure bug where "git ls-remote" without a repo
didn't support global url.*.insteadOf, even though "git clone" and
"git ls-remote" in any repo did.

Also, enforce that all callers provide a struct remote to transport_get().

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