transport: teach all vtables to allow fetch first

transport: teach all vtables to allow fetch first

The only transport that does not allow fetch() to be called before
get_refs_list() is the bundle transport. Clean up the code by teaching
the bundle transport the ability to do this, and removing support for
transports that don't support this order of invocation.

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