GUI call sites that need a base branch all share the same logic: ask
GetBaseBranchCandidates, take candidates[0] as the config-order
tiebreak, and surface the candidate list when the user needs to
disambiguate. Extracting that into a helper keeps the upcoming prompt
and rebase wiring focused on UX concerns. Not yet routed through —
subsequent commits replace the direct GetBaseBranchCandidates calls
with ResolveBaseBranch.
672a37031d