repack-midx: extract `repack_fill_midx_stdin_packs()`

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>