commit-graph: prepare commit graph

commit-graph: prepare commit graph

Before checking if the repository has a commit-graph loaded, be sure
to run prepare_commit_graph(). This is necessary because otherwise
the topo_levels slab is not initialized. As we compute topo_levels for
the new commits, we iterate further into the lower layers since the
first visit to each commit looks as though the topo_level is not
populated.

By properly initializing the topo_slab, we fix the previously broken
case of a split commit graph where a base layer has the
generation_data_overflow chunk.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>