commit-graph.c: extract `verify_one_commit_graph()`

When the `verify_commit_graph()` function was extended to support
commit-graph chains via 3da4b609bb1 (commit-graph: verify chains with
--shallow mode, 2019-06-18), it did so by recursively calling itself on
each layer of the commit-graph chain.

In practice this poses no issues, since commit-graph chains do not loop,
and there are few enough of them that adding additional frames to the
stack is not a problem.

A future commit will consolidate the progress output from `git
commit-graph verify` when verifying chained commit-graphs to print a
single line instead of one progress meter per commit-graph layer.
Prepare for this by extracting a routine to verify a single layer of a
commit-graph.

Note that `verify_commit_graph()` is still recursive after this patch,
but this will change in the subsequent patch.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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