repack: move `pack_kept_objects` to `struct pack_objects_args`

repack: move `pack_kept_objects` to `struct pack_objects_args`

The "pack_kept_objects" variable is defined as static to the repack
builtin, but is inherently related to the pack-objects arguments that
the builtin uses when generating new packs.

Move that field into the "struct pack_objects_args", and shuffle around
where we append the corresponding command-line option when preparing a
pack-objects process. Specifically:

 - `write_cruft_pack()` always wants to pass "--honor-pack-keep", so
   explicitly set the `pack_kept_objects` field to "0" when initializing
   the `write_pack_opts` struct before calling `write_cruft_pack()`.

 - `write_filtered_pack()` no longer needs to handle writing the
   command-line option "--honor-pack-keep" when preparing a pack-objects
   process, since its call to `prepare_pack_objects()` will have already
   taken care of that.

   `write_filtered_pack()` also reads the `pack_kept_objects` field to
   determine whether to write the existing kept packs with a leading "^"
   character, so update that to read through the `po_args` pointer
   instead.

 - `cmd_repack()` also no longer has to write the "--honor-pack-keep"
   flag explicitly, since this is also handled via its call to
   `prepare_pack_objects()`.

Since there is a default value for "pack_kept_objects" that relies on
whether or not we are writing a bitmap (and not writing a MIDX), extract
a default initializer for `struct pack_objects_args` that keeps this
conditional default behavior.

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