Shift object enumeration out of upload-pack

Shift object enumeration out of upload-pack

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>