Merge branch 'en/commit-graph-timestamp-fix'

Merge branch 'en/commit-graph-timestamp-fix'

compute_reachable_generation_numbers() in commit-graph used a 32-bit
integer to accumulate parent generations, which is OK for generation
number v1 (topological levels), but with generation number v2
(adjusted committer timestamps), it truncated timestamps beyond
2106.  Fixed by widening the accumulator to timestamp_t.

* en/commit-graph-timestamp-fix:
  commit-graph: use timestamp_t for max parent generation accumulator