midx: drop redundant `struct repository` parameter

midx: drop redundant `struct repository` parameter

There are a couple of functions that take both a `struct repository` and
a `struct multi_pack_index`. This provides redundant information though
without much benefit given that the multi-pack index already has a
pointer to its owning repository.

Drop the `struct repository` parameter from such functions. While at it,
reorder the list of parameters of `fill_midx_entry()` so that the MIDX
comes first to better align with our coding guidelines.

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