csum-file.c: extract algop from hashfile_checksum_valid()

csum-file.c: extract algop from hashfile_checksum_valid()

Perform a similar transformation as in the previous commit, but focused
instead on hashfile_checksum_valid(). This function does not work with a
hashfile structure itself, and instead validates the raw contents of a
file written using the hashfile API.

We'll want to be prepared for a similar change to this function in the
future, so prepare ourselves for that by extracting 'the_hash_algo' into
its own field for use within this function.

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