repo: replace get_value_fn_for_key by get_repo_info_field

repo: replace get_value_fn_for_key by get_repo_info_field

Remove the function `get_value_fn_for_key`, which returns a function that
retrieves a value for a certain repo info key. Introduce `get_repo_info_field`
instead, which returns a struct field.

This refactor makes the structure of the function print_fields more consistent
to the function print_all_fields, improving its readability.

Signed-off-by: Lucas Seiki Oshiro <lucasseikioshiro@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>