commit-reach: move min_generation check into paint_queue_get()

commit-reach: move min_generation check into paint_queue_get()

Consolidate the min_generation termination condition into
paint_queue_get(), alongside the existing stale-entry and
side-exhaustion checks.

Move last_gen into struct paint_state so that
commit_graph_generation() is called exactly once per dequeued commit
and the result is shared across all termination checks and the
monotonicity BUG assertion.

Signed-off-by: Kristofer Karlsson <krka@spotify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>