commit-graph: use timestamp_t for max parent generation accumulator

commit-graph: use timestamp_t for max parent generation accumulator

compute_reachable_generation_numbers() computes each commit's
generation as

    max(c->date, max(parent.generation)) + 1

by walking its parents and accumulating their generations into a
local

    uint32_t max_gen = 0;

while info->get_generation() returns timestamp_t and
compute_generation_from_max() already takes its max_gen parameter
as timestamp_t.  For v1 (topological levels) the narrowing is
harmless because GENERATION_NUMBER_V1_MAX is less than 2^30, but
for v2 (corrected committer dates) it silently truncates any
parent generation that does not fit in 32 bits, i.e. any parent
whose committer timestamp is at or beyond 2106-02-07 UTC
(>= 2^32).

The truncated max then causes child commits to end up with a
corrected committer date that matches the parent's instead of being
at least 1 higher.  The bad value gets written into the commit-graph
and causes problems later, and can be noticed by running `git
commit-graph verify`.

Widen the accumulator to timestamp_t.

This is solely an in-memory arithmetic fix with no on-disk format
change: the on-disk format already encodes timestamp_t values and
existing readers handle them unchanged.  This merely allows the code to
compute the correct value to write to disk.

The narrowing was introduced in 80c928d947c2 (commit-graph:
simplify compute_generation_numbers(), 2023-03-20), which rewired
v2 to use the shared compute_reachable_generation_numbers()
helper; the helper's local accumulator had been declared uint32_t
in the immediately preceding 368d19b0b7fa (commit-graph: refactor
compute_topological_levels(), 2023-03-20) when only v1 was using
it, where it was harmless.

Add a new test with a future-dated parent and a present-day child;
without the above fix, `git commit-graph verify` reports the
descendant's stored generation as below parent + 1.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>