repack-midx: extract `repack_fill_midx_stdin_packs()`
The function `write_midx_included_packs()` manages the lifecycle of writing packs to stdin when running `git multi-pack-index write` as a child process. Extract a standalone `repack_fill_midx_stdin_packs()` helper, which handles `--stdin-packs` argument setup, starting the command, writing pack names to its standard input, and finishing the command. This simplifies `write_midx_included_packs()` and prepares for a subsequent commit where the same helper is called with `cmd->out = -1` to capture the MIDX's checksum from the command's standard output, which is needed when writing MIDX layers with `--no-write-chain-file`. 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>