commit-graph: add trace2 instrumentation for generation DFS

commit-graph: add trace2 instrumentation for generation DFS

Count the number of steps taken in
compute_reachable_generation_numbers() and expose it via
trace2 to make it easier to detect performance regressions.

Add a failing test for such a regression, introduced in
199d452758 (commit-graph: return the prepared commit graph
from `prepare_commit_graph()`, 2025-09-04), where incremental
commit-graph writes do not see existing generation numbers
from lower graph layers and fall back to walking the full
ancestry.

Signed-off-by: Kristofer Karlsson <redacted>
Signed-off-by: Junio C Hamano <redacted>