transport: rename "fetch" in transport_vtable to "fetch_refs"

transport: rename "fetch" in transport_vtable to "fetch_refs"

Rename the "fetch" member of the transport_vtable to "fetch_refs" for
consistency with the existing "push_refs". Neither of them just push
"refs" but refs and objects, but having the two match makes the code
more readable than having it be inconsistent, especially since
"fetch_refs" is a lot easier to grep for than "fetch".

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>