midx: cleanup internal usage of `the_repository` and `the_hash_algo`

midx: cleanup internal usage of `the_repository` and `the_hash_algo`

In the `midx.c` file, there are multiple usages of `the_repository` and
`the_hash_algo` within static functions of the file. Some of the usages
can be simply swapped out with the available `repository` struct. While
some of them can be swapped out by passing the repository to the
required functions.

This leaves out only some other usages of `the_repository` and
`the_hash_algo` in the file in non-static functions, which we'll tackle
in upcoming commits.

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