commit-graph: verify chains with --shallow mode

commit-graph: verify chains with --shallow mode

If we wrote a commit-graph chain, we only modified the tip file in
the chain. It is valuable to verify what we wrote, but not waste
time checking files we did not write.

Add a '--shallow' option to the 'git commit-graph verify' subcommand
and check that it does not read the base graph in a two-file chain.

Making the verify subcommand read from a chain of commit-graphs takes
some rearranging of the builtin code.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>