midx: pass down `hash_algo` to functions using global variables

midx: pass down `hash_algo` to functions using global variables

The functions `get_split_midx_filename_ext()`, `get_midx_filename()` and
`get_midx_filename_ext()` use `hash_to_hex()` which internally uses the
`the_hash_algo` global variable.

Remove this dependency on global variables by passing down the
`hash_algo` through to the functions mentioned and instead calling
`hash_to_hex_algop()` along with the obtained `hash_algo`.

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