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>