midx-write: pass down repository to static functions

midx-write: pass down repository to static functions

In 'midx-write.c' there are a lot of static functions which use global
variables `the_repository` or `the_hash_algo`. In a follow up commit,
the repository variable will be added to `write_midx_context`, which
some of the functions can use. But for functions which do not have
access to this struct, pass down the required information from
non-static functions `write_midx_file` and `write_midx_file_only`.

This requires that the function `hash_to_hex` is also replaced with
`hash_to_hex_algop` since the former internally accesses the
`the_hash_algo` global variable.

This ensures that the usage of global variables is limited to these
non-static functions, which will be cleaned up in a follow up commit.

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