pack-objects: parameterize pack-reuse routines over a single pack

The routines pack-objects uses to perform verbatim pack-reuse are: - write_reused_pack_one() - write_reused_pack_verbatim() - write_reused_pack() , all of which assume that there is exactly one packfile being reused: the global constant `reuse_packfile`. Prepare for reusing objects from multiple packs by making reuse packfile a parameter of each of the above functions in preparation for calling these functions in a loop with multiple packfiles. Note that we still have the global "reuse_packfile", but pass it through each of the above function's parameter lists, eliminating all but one direct access (the top-level caller in `write_pack_file()`). Even after this series, we will still have a global, but it will hold the array of reusable packfiles, and we'll pass them one at a time to these functions in a loop. Note also that we will eventually need to pass a `bitmapped_pack` instead of a `packed_git` in order to hold onto additional information required for reuse (such as the bit position of the first object belonging to that pack). But that change will be made in a future commit so as to minimize the noise below as much as possible. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>