midx: compute paths via their source

midx: compute paths via their source

With the preceding commits we started to always have the object database
source available when we load, write or access multi-pack indices. With
this in place we can change how MIDX paths are computed so that we don't
have to pass in the combination of a hash algorithm and object directory
anymore, but only the object database source.

Refactor the code accordingly.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>