[PATCH v4 0/3] Reuse --contains traversal results

git tag uses a memoized traversal for --contains, while git branch and git for-each-ref repeat a reachability walk for each ref. Reuse the memoized traversal when generation numbers can bound the walk.

The first patch makes the memoized traversal reject cyclic replacement histories. The last makes the non-memoized path report reachability errors.

Signed-off-by: Tamir Duberstein <tamird@gmail.com>

---
Changes in v4:
- Die on cyclic ancestry instead of retrying another reachability walk.
- Update the cycle test and credit Kristofer Karlsson.
- Remove unexplained links to review messages.
- Link to v3: https://patch.msgid.link/20260611-ref-filter-memoized-contains-v3-0-b26af3dba285@gmail.com

Changes in v3:
- Split missing-ancestor error handling into its own patch.
- Use die() for reachability errors, remove redundant cache setup, and
  chain cycle-test cleanup.
- Drop the unrelated empty-target-list behavior change.
- Explain why git tag retains memoization without generation numbers.
- Add p1500 coverage for all three frontends and a shared-history
  case.
- Remove correctness checks from p1500 and drop output hashes.
- Link to v2: https://patch.msgid.link/20260608-ref-filter-memoized-contains-v2-0-e72720344a7c@gmail.com

Changes in v2:
- Split cycle handling into a preparatory patch.
- Exercise cycle handling through the existing git tag path.
- Move perf result verification out of setup.
- Link to v1: https://patch.msgid.link/20260607-ref-filter-memoized-contains-v1-1-a1972dde9c76@gmail.com

---
Tamir Duberstein (3):
      commit-reach: reject cycles in contains walk
      ref-filter: memoize --contains with generations
      commit-reach: die on contains walk errors

 commit-reach.c                 | 23 ++++++++++++++++++-----
 commit-reach.h                 |  3 ++-
 t/perf/p1500-graph-walks.sh    | 28 +++++++++++++++++++++++++++-
 t/t6301-for-each-ref-errors.sh | 22 ++++++++++++++++++++++
 t/t7004-tag.sh                 | 18 ++++++++++++++++++
 5 files changed, 87 insertions(+), 7 deletions(-)
---
base-commit: 9ac3f193c05c2237e2b14ebaa1149e9fc8a1abe0
change-id: 20260607-ref-filter-memoized-contains-7cb6b3bccad1

Best regards, -- Tamir Duberstein <tamird@gmail.com>