commit-reach: replace queue_has_nonstale() scan with O(1) tracking

paint_down_to_common() and ahead_behind() call queue_has_nonstale()
on every iteration to decide whether to continue the walk.
queue_has_nonstale() performs a linear scan of the priority queue,
making the overall walk O(n*m) where n is the number of commits
walked and m is the queue size.

Introduce 'struct nonstale_queue', a thin wrapper around prio_queue
that maintains a 'max_nonstale' pointer — the lowest-priority
(oldest) non-stale commit seen so far. When this commit is popped,
every remaining queue entry is known to be stale, so the walk can
stop. This reduces the per-iteration termination check from O(m)
to O(1).

Uses <= 0 (not < 0) when comparing priorities so that among distinct
commits with equal priority (same generation and timestamp) the
last-enqueued one is tracked. Since prio_queue breaks ties by
insertion order, this ensures max_nonstale is always the last in its
priority class to be popped, making pointer equality on pop
sufficient for correctness.

The previous commit's ENQUEUED deduplication guarantees each commit
appears at most once in the queue, which is required for the pointer
equality check to be unambiguous.

On a large monorepo (3.7M commits), this yields ~2x end-to-end
speedup for merge-base calculations on deep import branches.
Profiling shows paint_down_to_common() drops from 50% to 4% of
total runtime (~27x faster), with the remaining time in commit
graph lookups and heap operations:

  Before: 8536ms / 5757ms / 4743ms  (three test cases)
  After:  3956ms / 4383ms / 1927ms

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Kristofer Karlsson <krka@spotify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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