clone: support unusual remote ref configurations

clone: support unusual remote ref configurations

When cloning a branchless and tagless but not refless remote using
protocol v0 or v1, Git calls transport_fetch_refs() with an empty ref
list. This makes the clone fail with the message "remote transport
reported error".

Git should have refrained from calling transport_fetch_refs(), just like
it does in the case that the remote is refless. Therefore, teach Git to
do this.

In protocol v2, this does not happen because the client passes
ref-prefix arguments that filter out non-branches and non-tags in the
ref advertisement, making the remote appear empty.

Note that this bug concerns logic in builtin/clone.c and only affects
cloning, not fetching.

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