midx: pass a repository pointer

midx: pass a repository pointer

Much of the multi-pack-index code focuses on the multi_pack_index
struct, and so we only pass a pointer to the current one. However,
we will insert a dependency on the packed_git linked list in a
future change, so we will need a repository reference. Inserting
these parameters is a significant enough change to split out.

Signed-off-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>