commit-graph.c: iteratively verify commit-graph chains

commit-graph.c: iteratively verify commit-graph chains

Now that we have a function which can verify a single layer of a
commit-graph chain, implement `verify_commit_graph()` in terms of
iterating over commit-graphs along their `->base_graph` pointers.

This further prepares us to consolidate the progress output of `git
commit-graph verify`.

Signed-off-by: Taylor Blau <redacted>
Acked-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>