describe: use oidset in finish_depth_computation()

describe: use oidset in finish_depth_computation()

Depth computation can end early if all remaining commits are flagged.
The current code determines if that's the case by checking all queue
items each time it dequeues a flagged commit.  This can cause
quadratic complexity.

We could simply count the flagged items in the queue and then update
that number as we add and remove items.  That would provide a general
speedup, but leave one case where we have to scan the whole queue: When
we flag a previously seen, but unflagged commit.  It could be on the
queue and then we'd have to decrease our count.

We could dedicate an object flag to track queue membership, but that
would leave less for candidate tags, affecting the results.  So use a
hash table, specifically an oidset of commit hashes, to track that.
This avoids quadratic behaviour in all cases and provides a nice
performance boost over the previous commit, 08bb69d70f (describe: use
prio_queue_replace(), 2025-08-03):

Benchmark 1: ./git_08bb69d70f describe $(git rev-list v2.41.0..v2.47.0)
  Time (mean ± σ):     855.3 ms ±   1.3 ms    [User: 790.8 ms, System: 49.9 ms]
  Range (min … max):   853.7 ms … 857.8 ms    10 runs

Benchmark 2: ./git describe $(git rev-list v2.41.0..v2.47.0)
  Time (mean ± σ):     610.8 ms ±   1.7 ms    [User: 546.9 ms, System: 49.3 ms]
  Range (min … max):   608.9 ms … 613.3 ms    10 runs

Summary
  ./git describe $(git rev-list v2.41.0..v2.47.0) ran
    1.40 ± 0.00 times faster than ./git_08bb69d70f describe $(git rev-list v2.41.0..v2.47.0)

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>