midx-write: extract function to test whether MIDX needs updating

midx-write: extract function to test whether MIDX needs updating

In `write_midx_internal()` we know to skip writing the new multi-pack
index in case it would be the same as the existing one. This logic does
not handle the `--stdin-packs` option yet though, so we end up always
rewriting the MIDX if that option is passed to us.

Extract the logic to decide whether or not to rewrite the MIDX into a
separate function. This will allow us to extend that feature in the next
commit to address the above issue.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>