commit-graph: simplify compute_generation_numbers()

commit-graph: simplify compute_generation_numbers()

The previous change introduced the generic algorithm
compute_reachable_generation_numbers() and used it as the core
functionality of compute_topological_levels(). Now, use it as the core
functionality of compute_generation_numbers().

The main difference here is that we use generation version 2, which is
used in to toggle the logic in compute_generation_from_max() for
computing the corrected commit date based on the corrected commit dates
of the parent commits (and the commit date of the current commit). It
also uses different methods for (get|set)_generation in the vtable in
order to store and access the value in the correct places.

Co-authored-by: Taylor Blau <me@ttaylorr.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>