commit-graph: introduce `ensure_generations_valid()`

Use the just-introduced compute_reachable_generation_numbers_1() to
implement a function which dynamically computes topological levels (or
corrected commit dates) for out-of-graph commits.

This will be useful for the ahead-behind algorithm we are about to
introduce, which needs accurate topological levels on _all_ commits
reachable from the tips in order to avoid over-counting.

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

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