When a dependency task merges and its branch is deleted, self-healing nulls
the dependent task's baseBranch. Both resolveDiffBase (dashboard) and
resolveTaskDiffBaseRef (merger) defaulted to "main" in that case, widening
the diff range to merge-base(HEAD, main) and surfacing unrelated history —
e.g. FN-2855 reported 108 changed files instead of 16. Skip the merge-base
step when baseBranch is unset and a baseCommitSha is recorded; fall back to
"main" only for legacy tasks lacking both hints.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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