csum-file.c: use unsafe_hash_algo()

csum-file.c: use unsafe_hash_algo()

Instead of calling the unsafe_ hash function variants directly, make use
of the shared 'algop' pointer by initializing it to:

    f->algop = unsafe_hash_algo(the_hash_algo);

, thus making all calls use the unsafe variants directly.

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