Offload object enumeration in upload-pack to pack-objects, but fall
back on internal revision walker for shallow interaction. Aside from
architecturally making more sense, this also leaves the door open for
pack-objects to employ a revision cache mechanism. Test t5530 updated
in order to explicitly check both enumeration methods.
Signed-off-by: Nick Edelen <sirnot@gmail.com> Acked-by: Nicolas Pitre <nico@cam.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>