object-file: split out logic regarding hash algorithms

object-file: split out logic regarding hash algorithms

While we have a "hash.h" header, the actual implementation of the
subsystem is hosted by "object-file.c". This makes it harder than
necessary to find the actual implementation of the hash subsystem and
intermingles the different concerns with one another.

Split out the implementation of hash algorithms into a new, separate
"hash.c" file.

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