repack-midx: factor out `repack_prepare_midx_command()`

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>