commit-graph.h: use odb in 'load_commit_graph_one_fd_st'

commit-graph.h: use odb in 'load_commit_graph_one_fd_st'

Apply a similar treatment as in the previous patch to pass a 'struct
object_directory *' through the 'load_commit_graph_one_fd_st'
initializer, too.

This prevents a potential bug where a pointer comparison is made to a
NULL 'g->odb', which would cause the commit-graph machinery to think
that a pair of commit-graphs belonged to different alternates when in
fact they do not (i.e., in the case of no '--object-dir').

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