repack: move `finish_pack_objects_cmd()` out of the builtin

repack: move `finish_pack_objects_cmd()` out of the builtin

In a similar spirit as the previous commit(s), now that the function
`finish_pack_objects_cmd()` has no explicit dependencies within the
repack builtin, let's extract it.

This prepares us to extract the remaining two functions within the
repack builtin that explicitly write packfiles, which are
`write_cruft_pack()` and `write_filtered_pack()`, which will be done in
the future commits.

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