midx: expose `write_midx_file_only()` publicly

Expose a variant of the write_midx_file() function which ignores packs
that aren't included in an explicit "allow" list.

This will be used in an upcoming patch to power a new `--stdin-packs`
mode of `git multi-pack-index write` for callers that only want to
include certain packs in a MIDX (and ignore any packs which may have
happened to enter the repository independently, e.g., from pushes).

Those patches will provide test coverage for this new function.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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