csum-file: store the hash algorithm as a struct field

csum-file: store the hash algorithm as a struct field

Throughout the hashfile API, we rely on a reference to 'the_hash_algo',
and call its _unsafe function variants directly.

Prepare for a future change where we may use a different 'git_hash_algo'
pointer (instead of just relying on 'the_hash_algo' throughout) by
making the 'git_hash_algo' pointer a member of the 'hashfile' structure
itself.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>