midx: pass `repository` to `load_multi_pack_index`

midx: pass `repository` to `load_multi_pack_index`

The `load_multi_pack_index` function in midx uses `the_repository`
variable to access the `repository` struct. Modify the function and its
callee's to send the `repository` field.

This moves usage of `the_repository` to the `test-read-midx.c` file.
While that is not optimal, it is okay, since the upcoming commits will
slowly move the usage of `the_repository` up the layers and remove it
eventually.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>