Re: [PATCH v2 2/8] builtin/multi-pack-index.c: support `--stdin-packs` mode

An alternative approach
would have been to call that function from the `git repack` builtin
directly, but this introduces awkward problems around closing and
reopening the object store, so the MIDX will be written out-of-process.

I'm not sure if the implementation direction started by this patch
(eventually, running "git multi-pack-index write --stdin-packs" to
replace the midx of a repository while "git repack" is running) would
work on platforms in which mmap-ing a file means that other processes
can't delete it, but if it works on a Windows CI, this should be fine.
(I don't have a Windows CI handy to test it on, though.)

Assuming it works on platforms like Windows, this patch looks fine.