commit-graph: validate layers for generation data

commit-graph: validate layers for generation data

We need to be extra careful that we don't use corrected
commit dates from any layer of a commit-graph chain if there is a
single commit-graph file that is missing the generation_data chunk.
Update validate_mixed_generation_chain() to correctly update each
layer to ignore the generation_data chunk in this case. It now also
returns 1 if all layers have a generation_data chunk. This return
value will be used in the next change.

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