commit-graph: rewrite to use checksum_valid()

commit-graph: rewrite to use checksum_valid()

Rewrite an existing caller in `git commit-graph verify` to take
advantage of checksum_valid().

Note that the replacement isn't a verbatim cut-and-paste, since the new
function avoids using hashfile at all and instead talks to the_hash_algo
directly, but it is functionally equivalent.

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