builtin/repack.c: pass both pack_objects args to repack_config

builtin/repack.c: pass both pack_objects args to repack_config

A subsequent commit will remove 'delta_base_offset' as a static variable
within builtin/repack.c, and reintroduce it as a member of the 'struct
pack_objects_args'.

As a result, the repack_config callback will need to have both the
cruft- and non-cruft 'struct pack_objects_args's in scope. Introduce a
new 'struct repack_config_ctx' to allow the callee to provide both
pointers to the callback.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>