builtin/repack.c: remove "repack_promisor_objects()" from the builtin

builtin/repack.c: remove "repack_promisor_objects()" from the builtin

Now that we have properly factored the portion of the builtin which is
responsible for repacking promisor objects, we can move that function
(and associated dependencies) out of the builtin entirely.

Similar to previous extractions, this function is declared in repack.h,
but implemented in a separate repack-promisor.c file. This is done to
separate promisor-specific repacking functionality from generic repack
utilities (like "existing_packs", and "generated_pack" APIs).

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