commit-reach: terminate merge-base walk when one paint side is exhausted

Add an early termination check to paint_down_to_common() using the
per-side counters introduced earlier. Once the walk enters the
ordered region, terminate early when one side's exclusive count
drops to zero -- no new merge-base can form without both paint
sides meeting.

The check also waits for pending_merge_bases to reach zero, ensuring
all merge-base candidates have been dequeued and recorded before
exiting.

The optimization is gated by gen_ordered (which excludes v1
commit-graphs that use the date-ordering fallback) and by a
generation check against topo_ceiling. topo_ceiling is
GENERATION_NUMBER_INFINITY for v2 graphs and
GENERATION_NUMBER_V1_MAX for v1 graphs, so that saturated commits
are treated as unordered. Together these ensure the check only
fires in the ordered region where topological ordering holds.

The same topo_ceiling boundary is applied to the existing
single-result early exit so that all generation-dependent gates
express the same saturation-aware boundary consistently.

Step counts measured with trace2 on git.git with commit-graph:

  merge-base --all v2.0.0 v2.55.0-rc1:
    before: 72264 steps    after: 44589 steps

  merge-base --all v2.55.0-rc1 v2.55.0-rc1~5:
    before:   110 steps    after:     7 steps

Helped-by: Derrick Stolee <stolee@gmail.com>
Helped-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Kristofer Karlsson <krka@spotify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

0 files changed, 0 insertions, 0 deletions