repack-midx: factor out `repack_prepare_midx_command()`
The `write_midx_included_packs()` function assembles and executes a `git multi-pack-index write` command, constructing the argument list inline. Future commits will introduce additional callers that need to construct similar `git multi-pack-index` commands (for both `write` and `compact` subcommands), so extract the common portions of the command setup into a reusable `repack_prepare_midx_command()` helper. The extracted helper sets `git_cmd`, pushes `multi-pack-index` and a subcommand, and handles `--progress`/`--no-progress` and `--bitmap` flags. The remaining arguments that are specific to the `write` subcommand (such as `--stdin-packs`) are left to the caller. No functional changes are included in this patch. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>