builtin/repack.c: pass `repack_write_midx_opts` to `midx_included_packs`

builtin/repack.c: pass `repack_write_midx_opts` to `midx_included_packs`

Instead of passing individual parameters (in this case, "existing",
"names", and "geometry") to `midx_included_packs()`, pass a pointer to a
`repack_write_midx_opts` structure instead.

Besides reducing the number of parameters necessary to call the
`midx_included_packs` function, this refactoring sets us up nicely to
inline the call to `midx_included_packs()` into
`write_midx_included_packs()`, thus making the caller (in this case,
`cmd_repack()`) oblivious to the set of packs being written into the
MIDX.

In order to do this, `repack_write_midx_opts` has to keep track of the
set of existing packs, so add an additional field to point to that set.

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