The listing's per-branch [ahead behind conflicts] summary is cached in
memory keyed by (base-snapshot-id, source-revn, branch-revn), which
names exactly the inputs the computation reads. A stale key cannot
exist, so the cache needs no invalidation logic: a repeat listing with
no intervening save performs no comparison work, and a cache miss is
the old behaviour. Failed computations are not stored, so a broken
branch keeps degrading to zeros with a warning.
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